How else would you imagine you find the file to add?
Short answer : a file that is already playing.
Here are two examples where I would want to do this - I hope they will make my point a little clearer
1) A folder with a large number of files is playing - say a large compilation or a classical album with a large number of small pieces (in the case of classical music, possibly with similar names, possibly in a language other than English)
2) Play Doctor has generated a playlist, 100 random songs from the library.
In the above situations, when I hear something I like and I want to add it to a playlist, I should be able to add the song which is playing from Now Playing, so that i can access it easily later. I should not have to scroll through a hundred files, or navigate through the library to find the file and add it to a playlist
This is something quite basic which is possible to do in MC; I should be able to do it on a remote