What says on the title. Many people already throw this idea around, just keeping it alive.
Reason is simple: Doing this, it allow us to switch characters to run the raids with the characters we need, while getting loot for the characters we want. The most common scenario is when the raid need healers, but no one want to switch because they need item X with their rogue/fighter/wizard/whatever.
EDIT for clarification: Bound to Account on Acquire, Bound to Character on Equip