Primarily, syncing your documents between multiple Remarkable devices and their apps on your phone. The page on their site for it spells out everything pretty well.
I asked a friend who bought the new remarkable. He likes it but he says his old one was quite easy to link to google drive but the new one is not like that. Is it the same experience as you have?
Remarkable has not "stolen" anything back which they sold.
On the contrary, they grandfathered in users which bought a device prior to their charging for a subscription so that they all have free access.