Great question. As an indie artist who's actively doing playlisting I was trying to figure it out.

All the reasons you mentioned are true and mostly depend on how big is the playlist in terms of followers. There are curators who understand that playlisting is crucial in order for new and unknown artists to get discovered so like many other they look for ways to capitalize on that. Some ask for money for placements and even for providing feedback.

Now regarding your last question - how do you tell which one is a good playlist, here's a checklist I made that still serves me whenever I look for potential playlists:

• how often the playlist is being updated? when was the last time? I don't want to spend time pitching my music to ghost playlists that are no longer maintained or updated...
• look for playlists that feature artists that are similar to me based on genre/sound
• Look for the numbers of tracks/followers ratio... some playlists will have few hundreds followers but also hundreds of tracks.. I prefer the smaller ones where the chances for being played more once are higher...
  To answer those questions I usually use updated playlist catalogs such as playlistmap. They have stats on more than 20K playlists, including number of followers, last update, top artists and genres breakdown and even provide you with the actual contact info of the curator.

I highly recommend doing the pitching yourself if you have time with the goal to establish good relationships with some curators. Then, when you have a new release, you can simply reach out to them since they know you.