Any impact from behind is absorbed by the designed-in crash box, the "real" bumper. Panels are just facade. Behind the panels you have layers of structure, and depending on how big that "tap" was, it might have actually bent/altered the structure of several small pieces - lights, etc. If you get rear-ended by someone, you'll want your car brought back to its previous condition, no gaps, scratches, smudges or cracks. Beyond cosmetic appearance, you want to be sure that bumper can still hold up to a second impact at its original specs (10mph?).
That said, 3 grand seems pretty high for a "tap." But I'd expect anything beyond a bike to damage more than the $150 panel.