You're right, but it can be useful as most people have small sites when they do begin.
And it's always better to have it backed up in more than one place.
I'm also using dropbox but also email and I also saved it on an external drive.
You receive the backup in your email inbox, and once you receive your backup today and everything is fine, you can delete the older one. Because if you have a newer one, you can always revert to a fine version of your website should anything go wrong