[Jikanu]

Over your time, your computer accumulates fragmented files, especially when you download a lot of stuff. This starts to slow it down over time. Defragging programs organize the fragmented files, making your computer go faster. Windows computers come with a defragger, but there are better ones out there. I would reccomend IObit's smart defrag.

(Im not very tech-savvy, so i might've gotten facts a little bit mixed up there.)