Thanks for the clarification and the screenshot.
I believe when that option was initially introduced it was to backup MB's settings. Then Steven expanded it to allow other things to be backed up.
I don't use that feature to backup my music files as I believe it simply copies your files to the target location but doesn't check to see if files have been removed. It just copies. So if you did a backup when you had 10,000 music files and deleted 10 of them and ran another backup without adding any new files, instead of having 9,990, you'd still have 10,000.
As I mentioned previously a free app like SyncBack can be configured so that if doing a source to target backup you can set it so that if a file is on the target that is -not- on the source (because you deleted it, or renamed it, etc) it can delete it. The Pro version has lots of options for what they call "Decisions - File." I'm not sure if the free version does that. I'm not pushing it on you. I'm just letting you know there might be a better way to backup your music files.