I have so many reasons for kegging, but most of the mechanical universals have been mentioned already. It's simply the best package for most styles. I still bottle (from the keg) for competition and gifting and such, but rarely, and have a Blichmann gun that makes that a decent process as well. I just hate dealing with bottles no matter how good I get at it and how many years I've been doing it.
I generally keep 4 beers on tap and like the ability to sample a little bit at a time and serve small samples to visitors without having to open a whole bottle when not needed. It's surprising how many times a visitor will drink less than 12oz and sample all the beers, and most of the time it's not because the beer sucks :-0
I also like that when we have gatherings, brew days, parties, etc, that people enjoy serving themselves beer from a faucet. I see that for many folks it's a fun novelty that we take for granted. For kids we sometimes carbonate some water and make sodas with syrups. Parties don't end up with bottles everywhere and broken.