Well, there used to be a Spotify feature where you could tell it to generate a “radio” (algorithmic playlist) based on an existing playlist you have.

You have all your favorite songs of x mood in one playlist? Let the algorithm help you find more of this stuff!

Thankfully the execs at Spotify rightfully noticed that this was a good feature and immediately fucking axed it in the past year or so.

That was my favorite feature. That was why I used their damn service.