Playlists for visibility.
as well as actions like "show all tracks (in current playlist)"
those guys who have large sessions and lots of tracks feel me 100%
it is also very useful even in small sessions, to keep things clean
There have been a few threads on this and while after 2 years or so I've adapted to REAPER from Pro Tools, I still feel strongly that the one things Pro Tools does better than REAPER is playlists.
I don't see my mind changing on that until REAPER improves in this area.