So, that proto-bird's offspring would survive better than offspring of the others, and so eventually replace them. Fast forward a couple of hundred million years of the same variation, mutation, and selection, and you end up with modern bird feathers. The groove and hook did not evolve separately, as has been contended, but one in response to the other. The hooks were not useless without the grooves, they just work better with the grooves. You find this same pattern all through the living world.
Eyes are another example.
I'll now say a few unresearched things about eyes. Again, nothing could be further from the truth. Ask a Chlamydomonas algal cell, or a Euglena . They won't answer, but you can observe a very primitive eyespot on these cells. All it does is sense light.