There are many parts of the wings, especially wing panels and fairings at the wing root and around the engine pylon, that are made from a composite material (think fiberglass or similar). These areas can become damaged or eroded and the repairs need to be done with heat and vacuum. The tape is not there to hold the panel on, nor to beef up the strength.
The tape is there to prevent water ingression into the panel which makes the repair more difficult. Water ingression also adds weight to the airplane, which is another negative.