i reformated (and reinstalled win 7) on one of my laptops after 4 years to start from scratch. This is a quite old machine that
i'm also sometimes using for software tests, for testing registry keys and software settings etc. So i decided to reinstal everything (system/software): i wanted to have an absolutely 'clean' system again on this laptop... .
I'm also running two another laptops: both installations (windows and software) are 2-3 years old. I dont think its necessary to frequently reinstal the entire system. I guess it was really/absolutely necessary in the 90's (windows 95/98, Windows XP)....
...but now... ....i guess it isn't really necessary when running the latest OS's (like Win7, Win8.1, Win10)....