'While actually playing music it just isn't important that "Old Joe Clark" is mixolydian and "Douce Dame Jolie" is dorian. You just learn the song.'

But it might save a few Dissonances if the the musicians in the session were able to say, "Old Joe Clark? The mixolydian or the dorian version?"

As Jack said, this stuff can be useful as a means of communication. Several good posts here, thanks Jack, and as a melodeon player I second what you said about moothies and modes.