IIRC, two-stage triggers were orginally developed as a safety device for recruits.
In modern service rifle competition, you are required to have a trigger that can lift 4.5 pounds. This is pretty heavy for demanding target work. In a two-stage trigger, you can divide the weight between the two stages.
A two stage trigger ic commonly found in most older military rifles (M1 Garands, M14, 03, 1917, 98K etc). Basically you pull the first stage (2 to 4 lbs in competition) and the trigger will come to a definite stop. You can hold the trigger at this point for a long,long time. When the conditions are right, you take up the final amount and the shot breaks. Two stage triggers are famous for the break-like-a-glass-rod feel obtainable on the second stage. Since the weight required to finally break the shot is not too much, the net effect is a very light felt trigger pull.
HTH.....SRM