You should back up your computer regularly. You don't need to back up the whole system, but you should backup any files you created such as documents, spreadsheets, photos, financial info and media you paid for which you no longer can download such as music, videos, software installations or e-books. If you would be disappointed if you lost it, then you should back it up. The operating system and applications can be reinstalled but your other files would be lost.
Ideally, you should store these backup copies in a location other than where the source is located. It would be a shame to have a fire or flood and lose both your data and your backup. You may want to consider a cloud backup solution. Many are only a few dollars a month, or if you have a gmail account, you can back your data to Google Drive automatically, then you have access to it anywhere, free!