Can hail-damaged shingles be repaired, or should they be replaced?
The ceramic granules are the protective layer of the shingle. They're not merely aesthetic. They offer UV, impact, and corrosion protection, and the shingle is vulnerable when they're removed. 

The roof probably will not leak now, or in a few weeks, or a few months, but eventually the shingles will deteriorate badly enough that water gets through above the top edge of the underlying shingle, and then it can reach the deck and the attic. They need to be replaced. 

While you could feasibly get some high-grade roofing adhesive and some ceramic sand and individually repair each divot, the result will not likely last as long as the original shingle would have. It'll also tend to show from the ground, either as color variation or bumps.