Reasons to Group or Raid

Advantages of a Raid

Warleaders (the healing ones) and defilers become enormously effective being able to see health bars and use their group heal effectively.
Coordinated focus fire from over a dozen creeps will kill many targets even when being massively healed
Coordinated movement in a flowing battle reduces losses with stragglers
Voice and /raidshout are "in your face (and ears)" communication methods to get everyone together on a plan and execute it. Also when the situation shifts, communication is paramount to getting creeps to rally around the new threat
BA's, wargs, and weavers coordinating together with cripples, hinders, and stuns can turn a running freep into a dead freep.