Vibe, and purpose. I have a gym playlist full of metal, 90’s rap, and some bebop. I also have a playlist for rock, another for metal, a classical playlist, a medievalish playlist (think Danheim, Heilung, The HU, etc), and another for just jazz. I also have playlists for the decades spanning from the 50’s to the 90’s. Ended up doing playlists for whenever I’m feeling really good, and for whenever I’m down in the dumps, just in case.
The decades playlists really help with being handed the aux. Most people don’t do well going from Toto or Green Day to Messhuggah and Opeth, so, dividing a genre by decade is good. I know my grandma will not vibe with Polyphia, so I play her some latin music, classical, or jazz, and she’s fine with it.
This leads to many, many playlists, and there’s a lot of overlap, but I don’t really mind as long as I can make sure I have a playlist for any mood I might find myself in.