This reason is straight forward, you're noticing more and more crashes with your computer, perhaps it keeps not responding, In this instance
It is advisable to check your hardware: Make sure your hard drive isn't ticking (which means it's dying), replace it if you can and do a full format
and reinstall this way you've a clean install on possibly new hardware or a completely clean hard drive, this helps with fragmentation too.
It's always my firm suggestion that if you have had a virus on your system to format it, if your system has been comprimised it's hard to determine
to what degree so it's always a safe bet to do a full format and reinstall. Sure reinstalling takes time for a possiblity of a non-issue however at the cost
of maybe of a few hours of backing up and reinstalling can in turn save you hundreds (if not more) of dollars.
As the hard drive writes data it writes to different sections of the hard drive, using defrag attempts to move that information commonly accessed together.
So a defrag is good enough right? Well yes and no, if you factor in time it takes to defrag vs reinstall time as well as the stability you will get from a fresh
install then you'll probably notice that reinstalling once a year will probably work out better/more efficient.