Hopefully you’ll never need it, but having a backup of your web site can someday save the day (sounds like a song title, don’t you think?); there are some things that WordPress can help you recover — such as deleted or changed pages — but some important components — like plugin updates or image deletions — can’t be recovered easily within WordPress… and that’s where server backups come in handy.
As part of the hosting I offer my clients, I automatically back up, daily, everything to Amazon S3 “offsite” storage. (The “offsite” part means that if the server ever completely fails, I would still have access to the backups.) I keep 3 monthly backups, 4 weekly backups, and 7 daily backups available at all times (let me know if you want a more techy description of that part!), so you can go back in time in case things really get messed up.