Put it this way...backup is more for the scenarios where say you make a change on the server and something does not work right and you simply wish to restore to working condition, you can do the restore from latest working backup...link @Tal_Paz-Fridman also explains that. Migrate export (or migrate server) starting from R80.20 is what you would use if you wish to, for example, import all the objects and licenses, along with policy rules (packages), vpn communities, etc, into the new management server.
Andy