suggestion: a new option in MP3Tag, if enabled MP3Tag makes backup copies before modifying files.

pay-off:
(1) This serves as a backup function, in case something goes wrong.
(2) And this way, the user can compare the original source files with the files MP3Tag writes, in order to see what changes have been made exactly by MP3Tag.
(3) it is optional, so users are not forced to enable the backup function.

That would be of limited value unless you also want to keep multiple backups. With just one backup, as soon as you do a second save, you've lost the original file. And I find that doing multiple saves is commonplace. Change an album title and save all files in the album. Fix a typo in the album artist name, do a save. Run an action group. Run another. Save, save.

And, of course, it's going to be slow because you have to write the entire file from scratch each time you save even a minor edit.

There is no compare function in MP3tag to see what has been changed so the comparison would have to be manually file by file. I doubt that a letter-by-letter comparison would really be useful to anyone.
Also, after editing loads of files the extent of the collection would easily grow by gigabytes only due to the backups. If then disc space runs out it could be that not all modifications can be carried out which would then lead to an inconsistent collection.
If you need a backup function, use an extra suitable backup tool.

Only to get this straight: this is a freeware program which is miles better than most programs you have to pay for. I think this is due to the fact, that the program functions have been limited to manipulating mp3 files. There is no audio grabber, nor audio editor nor graphics editor nor file comparison - and all these functions have been requested before. So I would very much prefer it if MP3tag stays as clean as it is now and does not become the Swiss army knife for any task on the computer.

I suggest you consider something like SyncBack Pro (or the free version). I has several BU options.

Another options is something like Acronis True Image.

I use both programs for all of my BUs.