What is the purpose of trash mobs?

Q u o t e:
My question is, what is the purpose of trash mobs in instances?

Make them take longer?
Make them more "realistic"
Make them waste time?

Enlighten me.

Every great boss must have flunkies. It is simply more epic to have to fight your way through packs of enemies. If you just went from boss, to boss, to boss... it wouldn't be nearly as fun. Those additional mobs serve as a break between major conflicts and give you a chance to interact with the boss of that area without directly confronting him/her/it.

Q u o t e:
trash makes a boss look more like a boss

if there were no trash and all bosses, there would be no bosses

it's like 'when everyone is super, no one is super'

Or, the other "bosses" would eventually be known as trash before the boss.