What is the point of playing assasins when bruisers simply do the job better?

Assasins easily have the burst damage to kill a squishy but whats the point of a champ who can bust someones 250%of someones hp when all you have to do is 100%?

Bruisers (i should divers) have the burst to kill a squishy, (divers) have the gapclosers to get to them, they arent nearly as reliant on ults to dish out the damage and they arent useless like most assasins once they have killed their target. They generally have more utility and survivability (via buildpath or with the kit) than assasins.

So what is the actual point of picking an assasin over a dover besides the possibility of better waveclear?