Well, the Download/Upload sync uses Renpy Sync so you can upload and download save files to/from Renpy server, which allows you to transfer your save files to another device.
I tested it just now to transfer my save files from PC to my phone and it worked alright.
Of course, the upload and download sync buttons appear on the save and load screen respectively, because you need to load the save file first, before you can upload sync it. But that’s also the default layout.
Hopefully this helps. Further info