Why disc brake applied on front wheel in bikes?

Answer 1

Some bikes have disc brakes on both wheels, but the front brake is more important. During braking you'll encounter something called dynamic weight transfer, which basically means that more of the bikes weight will be supported by thr front wheel and less by the rear. As the front wheel gets heavier the friction against the surface increases and it'll be to brake even harder, causing even more of the bike's weight to rest on the front wheel. All this heavy braking means that the front brake needs to be better at dispersing heat and it needs to be more easily modulated, as it's very hard to recover if the front wheel would lock up.