Some people love them, others people just don't bother...it's a personal type of thing.
They offer room for an extra battery (extended battery life), they offer the ability to use AA batteries, they have a shutter release button, control dial and thumb buttons so that you don't have to contort your hand when shooting int portrait orientation. They increase the grip area of the camera.
There are off-brand models that you can get for your camera. Some of them are built in the same factory as the Canon BG-E2, and are pretty much identical but are cheaper.