It's the equivalent of an automatic bookmark in book reader apps. While playing songs in a playlist, there is a tiny bit of state information - primarily what song you're on. As soon as you play something else, that state is discarded. There's no way to automatically pick up where you left off.
For example, I really enjoy curated playlists, and some of them are really long. I'd love the ability to work my way through a playlist but also be able to take a break and listen to something else. Or to go to the gym and listen to other music while I'm there. Or temporarily stop and listen to a new song a friend shared. None of these scenarios work very well currently.
Spotify already stores and syncs playlist name/ID/owner/etc. All I want is for them to add 2-3 bytes of sync'd data - just store the current sort field, sort direction, and a song index. It'd be totally fine if it didn't remember playlist state if you're on shuffle play.
Can you elaborate? Seriously curious how that exactly would look like.