As for a reason? I enjoy the challenge. The satisfaction of backstabbing a medic with full Uber as they are about to roll into your team and denying that is addictive to say the least. Sure, I won't be able to carry the team alone, but the right spy in the right place can make all the difference in the world.
Pyrbroh. ( Or Just pyro if theres no engineers or newbie engineers ), Scoot, Medic, Ahnd Enginenere
Pybro/Pyro: I like to help my team out. Especially the engineers. .w./I like trolling people and do combos.
Scoot: I like being fast and geting intel ( if its ctf ) quickly and being able to dodge helps too.
MEEM: I like Giving.
Engineer: I like helping the team with sentries, dispensers, and things that make you go far really fast.
Scout in case you didn't know. Scout's both an extremely efficient class and an extremely fun class to play. More often than not, whatever a Spy can do, Scout can do better (sorry Spy mains but it's fact.)