Or learn how to maintain your system properly 
Files will get fragmented. Updates will fill your computer with old, useless crap. Programs will fuck up. Some people get malware.
personally i tend to reformat/reinstall winblowz every year or so (or sometimes more often) just to get everything all running nice again…
never have to do anything like that on my linux partition mind you…
I tend to distro hop quite a lot.
So you don’t run updates? Cos everytime you format you need them updates again..
And no, don’t delete that much, because I’m selective of what I save. Plus all my files lives on my external drive, so changes to pics/music/vid/docs/etc… gets stored there away from main os files.
plus i run weekly defrag, and have a registry one that actually works. Also find not using 3rd party equivalents of existing stuff helps keep it clean…
I install the latest stable version of the OS rather than an old one and upgrade from there.
Waiting for someone to say “get the latest macbook pro”
… I also find that my traktor cpu redlines i dont like it but it still runs smooth…
I haven’t attempted a reformat yet, but it’s kind of the last thing I want to do. Dealing with program reinstalls, backing everything up beforehand, etc. is all kind of a pain that I’d rather not deal with.
One more bump.