You are comparing chalk & cheese. The /mir switch has nothing to do with file attributes. The /m switch does.
/mir Mirrors a directory tree
/m Copies only files for which the Archive attribute is set, and resets the Archive attribute.
Switches such as the /m switch operate on the source to select which files RoboCopy chooses to work with on that occasion.
The /mir switch operates on the destination instead. It tells RoboCopy to update files on the destination from the source [where the source has newer ones] and to delete files on the destination that no longer exist in the source.
So if you RoboCopy source files abcd then you will have abcd on the destination as well
but if you later delete d from the source you will still have d on the destination.
Results after next RoboCopy -
d will stay on the destination if you RoboCopy with the /m switch
d will be deleted from the destination if you RoboCopy with the /mir switch
The /mir switch can be very useful.
1 I use RoboCopy /mir as part of my backup regime. It creates a complete mirror image of all my files on the external HDD that I connect for that backup. If my computers ever failed or the place went up in flames then I could restore all my own folders & files just as they had been at the time the last RoboCopy was made.
2 I also use RoboCopy /mir as part of my network syncing routine.