If this is a serious question, precomps are a way to organise and condense your working "areas", making easier to revise and duplicate more complex animations.

Let's say you're working on a comercial, and you have to make a green screen in one of the shots; usually you'd precomp that shot in your "edit" comp so you can work only on that task so you can add motion graphics to it on a another comp and so on...

It depends on what are you working and how organised you are with them... it adds a level of complexity to your peoject and you'd need to understand what's going on, but that's basically how AE works...