I like to use my computer too rather than tinker with it all the time. But over the course of normal use, a lot of garbage accumulates -- old documents that no longer serve a purpose, software that I tried but didn't like, an internet downloads folder in desperate need of a flensing -- just stuff that needs to go away.
It's not just about technical issues, freeing up hard space and making things work better. It's just about clearing out the clutter and starting with a clean, well-organized slate once in a while. So I always install a new OS by formatting and starting over. It takes more time, but being able to go through everything that's on my system and really decide what I need to keep and what needs to go is helpful on many levels so I think it's worth it.