The following reasons exist to have a mountain bike:

-To justify keeping the ski lifts running in the summer (jobs for local teenagers)
-To "shred" (this is a good thing in MTB-speak) the alpine tundra in your nearest wilderness area and then, if challenged, make a prepared speech about how "nature doesn't care about us so why should we care about nature"
-To yell at hikers for getting in your way
-Because you're too incompetent to ride anything smaller than a 2.25" tire on a paved road, most likely because you are morbidly obese
-Because you're a boomer brained microdick and you saw Putin on a BSO MTB once and that makes it the one acceptable kind of bicycle

Also, everyone I see at my LBS who comes in with a mountain bike looks like this guy, no wonder their disc rotors look like something you would see on a moto GP bike

Actually I thought of a good reason, you're a broke student and that's the used bike they had at the price you could afford, if that's you, then you're alright, but if you bought a NEW MTB you are a loser, you are not changing my mind about this