A player may come across multiple islands on Discover that they want to play, but they can only play one at a time. Adding a way to save the ones that interests them to check out later will be a nice feature to have. This became more evident with the addition of the sponsored row whose content typically change very quickly.
This feature is inspired by the Watch Later feature from YouTube. When one browses the web, it is possible to open multiple tabs, but that’s not available in Discover. That’s why I think the Player Later feature will be needed more in Discover than the Watch Later is in YouTube.
Obviously, one can now use the Favorite feature as a makeshift replacement for Play Later.