They have passive safeties.

1. Trigger safety (have to pull the whole trigger or the upper part of the trigger is blocked by the frame)
2. Striker/firing pin block - if you take the slide off and look toward the rear, there's a round chunk o' metal sitting there. That prevents the striker, if charged (round racked), from striking the primer on a round should some how it move forward without a trigger pull. The trigger, when pulled, moves this block upward and out of the way of the firing pin so that it can move forward to fire (i.e. you must pull the trigger for the gun to go bang). You can take the slide off and try to move the firing pin with your hand, and you won't be able to.... press that round chunk o' metal down and hold it and then you can move the firing pin forward.

Those are the two mechanisms you'd be relying on, which as I understand, are reliable - but they're still mechanical and nature and everything is prone to failure.

That being said, the human brain is prone to failure as well.... big time.... and mistakes happen. It's a combination of knowledge, training, safeties, what's between your ears, etc. that come together to help prevent ND's, but they can still happen. There's also things like the loaded chamber visual indicator (i.e. the hole on the top of the slide to look in and see if a round is chambered), and the sear deactivation lever for 'safe' disassembly.

I have a manual safety on my 9c, because I also have a very light trigger in there (apex, not the factory trigger, it too has a passive trigger safety) and it's a carry weapon.

Although there's varying opinion's on external manual safeties for "DAO" pistols, I like it. It stops the trigger bar from moving backward to disengage the striker block, and subsequently drop the sear to allow the firing pin to move forward and the gun to go bang.

It's just another level of precaution in this new shooter's mind, and is very easy to train for to make sure it's off when you need it to be off. I've found it's all in my grip, and is second nature to disengage the safety when I present. It's really up to the shooter to decide.

Hope that helps a little. I'm still learning myself, but I went with the safety and have no issues with it (also sits in a ccw holster no problem, don't notice it). You can take them off easily if you want, but if you get it with a safety, I personally wouldn't remove it. If you end up using your weapon, the courts could use it against you saying you deliberately removed the safety. They could also have no clue you took it off. Better safe than sorry in my book (no pun intended).