If the hammer is cocked first, it takes very little pressure on the trigger to then fire the pistol. This is useful if you are trying to get a carefully aimed shot off.
If you have to fire the gun without the hammer being cocked, it takes a great deal more pressure on the trigger (and travel distance on the trigger), which can throw off your aim. (But better than nothing if you need to get a shot off quickly.)
So it is basically a safety feature. A double action trigger pull that cocks and releases the hammer is less accurate but more safe, whereas a single action trigger pull with a cocked hammer is less safe (because it takes very little pressure to fire the gun) but more accurate.
With a double action trigger, you therefore have an option as to which behavior you want. With semiautomatic pistols, the first shot by default is typically a full double-action trigger pull, while subsequent shots are single action because the hammer is automatically cocked as the slide cycles.
With revolvers, you have to manually cock the trigger each time if you want a single-action trigger pull. Old revolver designs were single action only, so you always had to cock the trigger, whereas most modern revolvers can also shoot with a double action trigger pull.
We are talking about the weird two trigger business. I’m fully aware of double/single action on revolvers. But some semi auto’s that have an indernal hammer have one trigger stacked on top of the other one. I’m talking triggers, not hammers. It just seems strange.
Glock’s Safe Action® System consists of three safeties, including the trigger safety you’re referring to. Glock says,
The trigger safety is designed to prevent the pistol from firing if it’s dropped or if the trigger is subjected to any pressure that isn’t a direct firing pull.
Not gonna login to Glock. But It makes no sense. It will only get jammed by a glove or something. Thumb safety is best. Like the 1911. Why just stack two triggers on top of each other?
How does it know that your are not intentionally depressing the trigger. When depressing the trigger releases the safety that releases the trigger.
It’s very, very dumb. Push on the trigger ‘BANG’ The safety doesn’t do a thing. It’s right on the front of the trigger. The safety is released when you pull the trigger.
The big reason to have it is so you can keep “one in the pipe” and be ready to fire instantaneously. You don’t have to fumble with (or remember!) the thumb safety. Good for people like cops that may have to fire quickly. Yet it keeps the gun safe from firing while just carrying it. In a 1911 the hammer has to be back to fire. So you have to cock it before. I’m not going to carry a 1911 with one in the chamber and the hammer pulled back, safety or no.