I do it about every 6 months. It's just a personal preference. I like cleaning out all the "ghosts" and "boogers" that accumulate from various sites and installs that never really delete. I save all the files I've collected to CD or DVD, then run DBAN to completely wipe the drive before re-installing the OS. As far as "tweaks", I've got all of my tweaks and notes on one CD that goes in right after the driver disc...so, re-tweaking to where I was before I started doesn't take long at all. (As a matter of fact, I'm here today because I just did a fresh re-install and need current versions of CCleaner and Speccy. ) Anyway, like I said, for me, it's a personal preference.
Still...it never fails...every time I do this, it feels like I've got a brand new computer without actually buying one.