a metronome is used to keep time, it wont mechanically make your rhythm "in time" you have to follow the beat it lays down. it's basically a clock that (hopefully) doesnt chyme!
they are used in recording studios so that live bands have a constant beat they can follow, so they dont speed up or slow down during the recording process.
rap and electronic artistes who use sequencers etc dont need them as the percussion sounds are mechanical and programed.
having said that....keep music live
phill