This is something I miss in SM Player (and in other players like VLC).
Why is a docked playlist useful?. Well, it is not if you're playing a movie. But it's a very nice thing for tutorials.
Tutorials use to come in short 5-10 minutes videos, each one covering one specific task. Best way to follow them is with two monitors: in one of them, you play the tutorial, in the other one, you work with the software/language and replicate what the guy is doing in the tutorial. It would be nice to have a docked playlist so you can jump easily between them.
And I suppose other people could find it interesting, like people that like to play music videos.