As was said above, it's a handy device if the music you want to play is on a sheet and you don't know what the tempo/speed should be.
Yes, that originally was Maelzel's idea, so appreciated by Beethoven that he dedicated M. a round (musical term, not in the pub).

A metronome is of no much use for me: I need 114 (German march), but the machines only give me 112 or 116.