Btukfyl

The world that I'm building is set in a fantasy world that is reminiscent of the early 20th century. Magic involves the traditional eight schools and anyone can become a magician if they choose to dedicate enough study and practice to it. Also, magitech--which only mages can use--exists alongside mundane technology.

Some ideas for mages using a gun could include enchanting them to fire infinite bullets (which when I think about it, would probably not be legal in my setting), putting spells on bullets to guide themselves to their targets, and imbuing them with elemental damage. But the question that I have is how would mages find guns practical when they could already sling around fireballs and such?

You ask

how would mages find guns practical when they could already sling around fireballs and such?

Well, in our world we have guns, too, but sometimes people use blades or even steel wires for scopes where the guns could still be used? Why?

Detectability and circumstances

There are tactical situation where using a certain type of weapons is not advisable. If you are a hit-man and want to kill your target intruding a highly guarded location, the loud bang of a weapon firing might make the job more difficult, so a blade would be preferable.

Same goes for magic: if there are ways to spot the usage of magic, using guns might be a good way to pass unnoticed, or even fool the opponent into believing something false (A is using guns, thus no risk of fireballs), which can be a tactical advantage.

Perhaps in your world magic is easy to see coming and not terribly hard to protect against, if you're trained to do so.

Bullets are a lot harder to protect against because they're very fast and hard to see and physically quite powerful. Some magical worlds allow people with the proper training to put up bulletproof shields, but this may not be possible (or easy) in your world.

Fire is messy, bullets are clean(er).

While a bullet can go astray (if one doesn't use magic to guide it), it won't cause harm to anything unless it hits it directly. Secondary effects are rare (like a bullet hits something hard enough to make it fall down). Generally, your bullet either hits your target or it doesn't.

Fire though...fire can light things on fire. If you miss with your fireball, you can cause a lot of destruction. The curtains catch on fire and the house goes up in flame. The fireball lands in a vacant lot that needs to be weed whacked and, an hour later, the neighborhood is on fire. Maybe your magic can stop it, maybe you're too busy fighting the person you just failed to kill.

Even if your fireball hits its target, you can cause a lot of destruction. The purpose of a fireball is to catch something on fire. It's hard to limit its scope. Then you have ash and embers and it's a big expensive and dangerous mess.

Who needs that hassle? Just get a nice pistol or shotgun and go take care of business.