I meet far too many people who don’t back up their computers.
It’s OK not to back up your computer if you don’t care about losing your data. If you don’t mind losing some things, like priceless photos forever, or having to recreate other important data from scratch, go ahead, don’t back up.
If, on the other hand, you do care about such things, here are steps on how to format an external hard drive for use as a Backup drive. Once the drive is set up, use software like SuperDuper or Carbon Copy Cloner to back up your computer to the newly formatted drive.