A backup is just the copy of your original site through which you can restore your original site if anything goes wrong. Keeping a backup on the safe side is always advisable.
A site is generally made up of two parts. One is all the installed themes/plugins, media and other is Database which stores all your blogs, posts, and comments. Without files, there is no site, and without the database, there is no data. Hence both of them is important and need a backup.
