Simple really…
I’m browsing Playlist A and it has some songs that I want to add to Playlist B. I click the circle to fill it in yellow on the tracks I want - just as I would do any other place in the Plex Web App - but then the only option is to remove them from the list. This seems very limiting as sometimes (in my case at least) it’s helpful to be able to browse certain playlists when creating a new one. Of course this can be done one at a time now, but on a large playlist this is not very convenient.