Lots of reasons. Sometimes to help me sleep before bed. Sometimes I get so turned on by something, wife isn't around or isn't feeling up to it so I have to get that tension out. Sometimes you just want to, and I'd rather do it than have sex, that's kinda rare but occasionally I get a huge urge to load up some porn and go at it instead of actual sex.
I don't do it as much as I used to because I prefer to save it up for a huge moneyshot for the wife.
Still get the urge constantly though...