Precomposing is useful because you can apply effects or animations to several layers instead of trying to do it on all of the layers individually, making it extremely difficult to get right and just making it slow. Precomposing also helps organize things into groups, so that your project isn't a huge pile of layers. There are also many different things you can do with pre-comps, like making proxies. You can go into the precomposition layer and make changes to animations inside there, and if you have the precomp in another composition, those changes will then be applied as well in the regular composition.
Essentially, it's just a faster way of making a new composition with selected layers.