Mainly for evidence in the event of an accident. Unfortunately we now live in an "Where there's blame there's a claim" society and there are people out there that orchestrate accidents to look like you are in the wrong. A recent one being flashing you out at a junction and then ploughing into you broadside "He flashed me out officer"..."No I didn't you just pulled out in front of me".
Only last night we went to a memorial for a local chap who had been killed in a knife attack, as we drove down the road a school lad on a bicycle came straight out in front of the car in front of us, the lad never looked, never stopped and literally I don't know how the girl driving the car stopped without hitting him. If she did I'm sure it would have looked worse on driver rather than the kid if there where no witnesses or video footage.
Some insurance companies will also give you a discount if one is fitted.
There are some drivers which won't like to have one as they generally drive like arseholes up the M6 in the 3rd line flashing their lights to get past everyone because they are quiet obviously more important that the rest of us and their reaction times are so good that they can drive 3 inches off your bumper.
Even if they do have them then at least the emergency services will have an idea of what happened after they've first scrapped them off the windscreen with a small putty knife.
EDIT:
Depending on which cam you get, most only record when you start the car but others record continuously. I recall earlier this year a guy parked his Aston Martin up and the cam was recording and they caught a guy keying the car. Police viewed the footage and got him.