LogFAQs > #967218523

You get the components that you want, you can pick higher quality components, and everything is standard so there are no surprises later on when you need to upgrade or repair. It will generally cost less for high end components.

Down side is no real warranty, you can totally wreck expensive parts if you don't know what you're doing, there's no guarantee all the parts will work together property, and it's time consuming to do the assembly. A big manufacturer may have access to cheaper low end components than you.