Let say my friand and I are living apart and want to listen to the same music, at the same time. It's a really easy feature to implement and I, a loner, spent nights with my friend chatting, listening to the same music but with delay. Let one user to be administrator, and anyone would be able to join his/her feed, listening to the same. I think it has a lot of potential in terms of marketing and all, influencer can create big communities, like the empaty boxes in 'Does Androids Dream of Electric Sheeps?'. It could create the community, or social media if you will, feeling that Spotify definitely lacks.