Playlists let bungie do a few things.
1) They remove host control and the ability for a host to take their ball and go home
2) since the criteria of game selection is less granular, they can focus on matching people based on other metrics, like Skill and connection.
3) When bungie wants to change a map/gametype that isn't working (say changing the arena zealot) they can control the change, and make sure it's applied consistently.
4) It helps expose the community to modes/gametypes that they might not have played before
5) You go into games knowing what to expect, you aren't going to pop into a game that is suddenly "High speed invasion splatters"