I walk to the Metro and take it to work. That's as far as I go. There must be a reason for metronomes but I haven't found it. And we're not alone--Brahms hated them--said something to the effect that he refused to accept that a machine could successfully reflect the soul of music.

Only thing I could possibly think of as a use would be to try to envision how fast a composer intended a piece to be played or sung since there are markings sometimes at the start of a piece indicating how fast to set the metronome. But only if you want a steady beat--which therefore knocks out any chance for interpretation through tempo--and as a result can kill the soul of a piece.

I'm convinced it should be up to the performer to decide how fast he or she wants to do a piece. The composers are mostly dead anyway.