Sometimes, I want to connect with people on Spotify that are not on Facebook (eg, my kids) but I cannot find them as users (on app, at least). All I can find, are my own Facebook friends who are connected to Spotify with FB credentials). My daughter would like to use her own Spotify account but all her hard music collection labor has been done with *my* account. Now, if we could connect, then I could just share her lists from my account with her account and she can listen regardless of where the list originated.
Also, sometimes you just want to listen to someone's music without becoming Facebook friends.