In reply to Jon Read: Convienince really. I can stand in a group photo, and shot from a reasonable distance, without having to lug 10 metres of cable about with me. Also, cables can cause shake if not managed right.
In reply to stroppygob:
I'm sure you're aware of having two devices: a wired remote timer, and a wireless shutter release. Both can be procured for not much. Not sure I've ever seen a wireless device that can do timed releases.
I have a wired remote I'm selling which does all of the above, except it is of course wired. Its a Aputure Timer Camera Remote Control Shutter Cable. Its very good, the best cable realise I've used on the Canon, only selling as it doesn't work with my Nikon's...