The reasons for a safey are numerious indeed. Not only in condition one, but if you're moving from cover to another position, it's a good idea to put the safety on while you do that. Climbing in and out of windows, all sorts of activity where you don't want to unload the gun, because you're in the middle of using it, but need to execute whatever you're doing safely.
You could ask the same question about why a safety on nearly any gun. Why would you need one on a bolt-action? Why would you need one on a lever gun? Why would you need one on an M16 (after all, the internal trigger parts are just a single action)? Why would you need one on a shotgun? Some DA/SA, and most DAO guns, and most revolvers don't have external safeties, but they have internal ones and are designed with this in mind.
If you're lowering or cocking the hammer for some reason and your thumb slips, then the half-cock will most likely stop the hammer from falling all the way to the firing pin.
Ross