It's typically referred to as a "half cock" in case you are looking for written references.
As above, it's a legacy of being a one-handed firearm meant for troops mounted upon a horse. That, if the hammer slipped from under a thumb, it should (ought) stop before striking the firing pin.
It's also meant to keep the hammer from following forward if the slide is retracted far enough to strip a round into the chamber--say, while holstering at the gallop or the like.
It's very much a late 19th century notion/idea, and largely obsolescent in the 21st century. But, held over in modern construction because it works as it was intended.
One of the better ways to learn about the 1911 is to look at its imitators and see what they kept and what they deleted. The Star and the Balister-Molina; to a lesser extent, the Polish Viz (better known as the Radom).