metronomes are useful for practise for certain set dances like the blackbird, garden of daisies, rodneys glory, these solo dances are normally played at a particular precise speed for competitions, it therefore makes sense to practise them at the speed that is required for competitions, the same goes for solo hornpipes and slip jigs and double jigs and reels when these tunes are going to be used for solo dancers.
for example solo slip jigs 113, solo hornpipes the same, garden of daisies 138 etc