It lets you download a ZIP archive with all your Set Lists, Part Lists, etc. in one click. This archive is convenient for backup purposes, but it is also in a format that can be exported to https://setmaster.pryp.in/
Additionally, if you click "Build with SetMaster" on a page of a Set, MOC or Part list on rebrickable.com, you will be conveniently redirected to a "build" page at https://setmaster.pryp.in/ without needing to do a manual export + import.
