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For the same reason some do in real life. Ship (or landing pod) landing isn't perfect so when they arrive back on earth the astronauts need to defend themselves against hostile animals or scavengers.

Warning: Dark

Imagine a situation where you're in your spaceship, you've run out of fuel, your comms are down, and you're flying through space away from civilisation.

You may have rations to last a few weeks, but with every passing hour, the chance of rescue gets slimmer and slimmer.

After a few days, you know there is no hope of anyone finding you.

You reach across to your firearm...

If you are not worried about Earth Wilderness landing like Leo Adberg has suggested (which is a very good real world answer), then an alternate would be to consider corportate/national espionage...

If there are only the biggest corporations and governments in space then its safe to assuming there is no space police, so if one company decided to hijack another's ship to either steal the presumably cargo, or possible find out what tech the rival company is working on, then its safe to assume the pilots would want to protect themselves.

Writing in that either companies or governments are in a type of cold war as to who can be the front runner in space travel and its not unreasonable to believe that with all the communication dead zones are the system (dark side of the moon for example, at least until china sent there probe a few weeks back) and there's plenty of areas where a ship could get be hijacked without anyone knowing.

Thankfully we've managed to avoid taking warfare to space so far but its not exactly out of character for the human race to do so...

No good reason for purpose-designed firearms

Risk assessments are critical to space travel, even more than on Earth. Some of the obvious things that can go wrong because you are carrying a firearm are:

• propellant becomes unstable


• poor maintenance results in mechanism seizing up


• accidental/negligent discharge results in personnel injury or death


• accidental/negligent discharge results in destruction of critical spacecraft components (including atmospheric integrity)


• accidental/negligent discharge imparts momentum (linear or rotational) to spacecraft requiring fuel expenditure for course correction

In addition to these possible risks there is also the certainty that every single flight will be wasting mass on a dangerous object of negligible use - mass that could be used to carry more fuel or operational/profit-generating payload.

To offset these massive disadvantages, there has to be a concrete benefit to putting a firearms on a spacecraft. This is hard to see - in the anti-hijacking role weapons such as tasers are much preferred to things that punch holes in your own ship. Boarding actions are dubious in any hard science setting - docking with a cooperative target is hard enough, docking with an uncooperative target is practically impossible even if its propulsion systems are disabled. As for employing firearms (other than those mounted in sophisticated turrets) against other spacecraft - forget it. Then there is the problem of keeping all of the astronauts current in their firearms training - if they are rusty then they may as well not have a firearm.

But wait...

Not all is lost, however. As was well-portrayed in The Martian, astronauts are a technically competent group of trained improvisers. If the plot of your story requires that a firearm is used then a character can improvise one. Hand-held and/or drone thruster units are a plausible item to have on board a spacecraft - if you disconnect a few safety devices and plug the exhaust with a ball bearing (possibly with some tape wrapped around it to get a good seal) then you have a projectile weapon. A high-pressure air tank is a ready-made reservoir for a very dangerous air rifle, just add a metal tube for a barrel and a valve. (With more time a semi- and/or full-auto version could be manufactured.)

If the requirement is just for a ranged weapon, not necessarily a firearm, then slings are just as easy to make as on Earth. A spacecraft machine shop could be used to make a bow or crossbow.

In summary - just as in many situations on Earth, carrying a firearm is far more dangerous than not carrying a firearm. However, spacecraft have plenty of options for making ranged weapons in an emergency.

One potential explanation (although not 100% satisfactory) is that it could form part of a survival kit. Bomber pilots, for example, were often equipped with survival kits including flares, emergancy rations and rifles such as here and
here, to help them forage for food and protect against wildlife should they crash land in remote areas. This could feasibly also apply to a spaceship, should they crash land on a remote part of earth.

There's two possible reasons I can think of.

The first is for some sort of emergency situation - much like trains have hammers to break the windows in an accident - perhaps there could be some situation where they'd need a weapon (are the weapons traditional current conventional weapons?) to break free of a crashed pod or something? Certainly you could argue the carrying of flare guns for situations like this - as safety equipment

The other is if there are any tensions between the big companies and/or wealthy nations. They could then be argued as necessary to protect a claim on a certain sector. Perhaps there had been an incident in the past where one company took over another, so they all carry an stock of weapons for protection against this. (easy to argue it's similar to countries on earth with weapons that are they will never use)

In space, accidents happen.

Just start some rumor, that space pirates/aliens/spies (or whatever fits your story the best) are responsible for those accidents (like missing people/spaceships). Until the investigations are closed and the results satisfy your astronauts, everyone starts to carry firearms "just in case". You can make those investigations really long, or maybe the custom to carry can stay.

You can also make one legitimate incident, where some madman started attacking his coworkers, and blame guns on him (shootings happen on Earth all the time), also an incident like this might be the best reason to introduce those checks in the first place.

Imagine that for some reason you are stuck in the middle of a microgravity environment, with nothing within your reach to pull or push yourself and start moving and no way to get outer assistance.

Your only hope to get in motion is to use reaction: expel some mass by exerting a force on it and, by reaction, have the mass exerting a force on you.

A gun with the jolt it provides upon firing is a reasonable way to use this principle. You might want to avoid armored shells to avoid damaging the hull and yourself with ricocheting bullets. Dense liquid filled bullets which splash on impact should do.

Security

Due to security reason all personnel have to go through thorough screening and checks

These checks do not end all of the possible security issues. They just ensure that a single individual or ar small group does not pose a significant threat.

But what if a group of individuals become the threat? Physical force is still a thing, and if a number of individuals mutiny or revolt, they could be dangerous, either by attacking the loyal crew or accessing critical parts of the ship.

Not to mention that people can become very imaginative when it comes to improvised weapon design.

To put an example, we already have places were people are very careful screened to avoid them inserting weapons in a controlled environments. Yet improvised weapons are made (shivs and the like), revolts do occasionally happen, and the guards do have weapons and/or have support from armed security forces readily available.

Imagine the situation in a spaceship where external support is not available and everyone on board knows about it.