1. can apply effects to loads of different channels at once (e.g have all your drum channels outing to a drums bus then you can put plugins on the bus to effect them all and you can change the volume of them all together etc)
2. reduce cpu workload (e.g all your drums outing to a drums bus, all your bass outing to a bass bus, and these then in turn out to the master. that way you've only got a few channels going to your main output rather than shit loads)
3. good for reverb, rather than putting reverb plugins on individual channels i like to have a send from the channel i want to effect to a reverb bus and control the amount of verb by changing the amount of send, that way you've still got dry signal coming through
4. can be handy for splitting up your bass i.e take a send from a bass channel, hi pass, add some distortion, chorus etc to make the top end more interesting.
Groups/FX Racks in Ableton do the same thing and more, esp useful as you can combine groups with diffrent routings, allot of creative shit can be done! just be careful not to feed anything back on itself without some kind of control