"I just want them to make one with a damn thumb safety and if this competition won't make them do it, nothing will." - some US Army ordinance guy about Glock, probably.
"Cocked & Locked" is usually how people refer to this - and it is easy to screw up. Under stress, people's fine motor skills vanish, sometimes resulting in the safety not being disengaged as you draw from the holster. Additionally, it can be accidentally flipped off during handling.
Modern firearms have multiple internal safeties to prevent accidental discharges (unless you're Sig apparently).
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.