If for some reason you're open carrying in a holster (like perhaps a police officer or soldier would be), and someone tries to get your weapon off of you and succeeds, a manual safety could save your life. They probably won't realize that the safety is on, and when they point the weapon at you and pull the trigger, nothing will happen, giving you a chance to escape or fight back.

Without that manual safety, the weapon just goes off and you now have an aftermarket hole installed in your body.

There's less of an argument manual safties in concealed carry, though. The opponent shouldn't know you have the weapon until it's drawn, so there's less chance of them getting it out of a holster.