You're talking about extended warranties.
manufacturer's usually have warranties(you don't have to buy separately), but you'd have to deal with the manufacturer, and not everything will be covered. With these extended warranties, they can cover more things, extend the life of the warranty and you'd be dealing with a warranty company and not the manufacturer.
The individual parts have warranty manufacturers warranties (retail box processors are usually 5 years), etc.. If you have a system built by a company, they will usually offer a warranty on the entire system (1-2 years possibly) and sell an extended warranty.