Do I need to bother using a database/file backup plugin if I'm already backing up my entire site using cPanel (WHM)?
Is there a disadvantage either way or are two different tasks at play?
If you are already backing up your website in cpanel (both database and files) then no, you will not need any plugins. However, you will need to access the website through cpanel/ftp to restore the backup.
But if you want to restore the backup in the back-end, you can find some plugins that will do the backup/restore without the need of accessing ftp or cpanel.
In overall, they both serve the same purpose.