On the smaller tanks <30g they get a 20-50% change every other week or so, mostly because 5 gallons is easy to tote around. For the 55g I was doing 10-15g water changes every couple weeks. On the large tank > 250 total it gets a 35 gallon every couple three weeks.

They are important to replace the trace elements that are consumed by various coral/critters and others that are skimmed out of the water column.