When people start lifting weights they are working towards building up their upper body strength. They want to get more strength in their arms. This is what lifting weights does. It allows people to build muscle and gain a greater endurance for lifting even heavier weights.
This is why weightlifters start with lifting bars with a couple of stack weights that are less than 50 pounds. As time progresses these weightlifters will slowly start to inch towards 100 pounds. They build and stack more weights as they become stronger. Eventually these people that started with weights that weighed less than a hundred pounds are able to lift hundreds of pounds with the bar for the weights if they continue to build their endurance.
This is something that is practiced by bodybuilders and general weightlifting enthusiasts. It helps define muscles and it provides people with a way to gain strength in their arms. People have the ability to condition their upper body better when they engage in weight lifting exercises.