Its not neccessary, but it depends on what your doing with comp. for example I have 3 comps. One for my mom, one laptop, one desktop, I've never had to reformat my moms cuz its never had any probs and if she gets a virus and it does no noticable damage than I just leave it alone (btw all she does is play games on it) My laptop I had to reformat often because I was downloading alot of diffrent programs which had cracks (don't do that any more). My desktop I've had for a few months now and will backup and restore every now and than. It really depends on what your computer is used for and if your data is important, which is up to you to choose.