A Group Session host can share a scannable Spotify Code from the app, giving whomever they want to share with full playback controls - pause, play, skip, select, and add tracks to the queue. Any added or removed songs will update on everyone's devices whenever connected. Group sessions will also end after an hour of inactivity.
This adds appeal and is a good perk for paid premium users, as it's always better to include additional features for higher-tier subscriptions to compel people to pay and buy. I also pay and use Spotify Premium, and while I enjoy offline listening and its smooth integration with my car's Apple CarPlay, personally it's nice to have more features which free users don't have. You know, like really putting that "premium" branding into use.