Its a thing, that brings life to a raid. We have even had a number of raids, whos number of trash pack can be counted on 2 hands, like BWD, BoT, OL, ML, GL among others. We have actually had a surprising number of raids, where the number of trash you had to interact with were very little, but god is it important that it is there.
Raids with no trash breask the illusions and shows clearly, that most raids are just different battle arenas with a big mob in the middle. Trash conceals that and makes it feel like place in use and living.