Tape prevents the migration of air from the high pressure side of the wing (or tail) to the low pressure side. This "migratory" air flow increases turbulence over the control surfaces which reduces their efficiency. It also increases overall drag & it can induce control surface flutter --- all of it bad stuff.

You don't have to use tape -- you can seal the surfaces with monocote (or whatever covering you are using). In fact, you don't have to do it at all -- but your birds will fly better if you do.