From what I understand, it kills off the beneficial bacteria, and I think doing that kinda re-cycles your tank. Get a second oppinion though just in case I'm wrong
tap water has added floride and other chemicals designed to kill bacteria
if you wash your filter media under the tap you will never have a properly matured tank
take alittle water out of you tank and rinse your filters in that the only part you can rinse under the
tap is the impeller (motor section) to wash the slime off