Why do sharks have nostrils if they breathe through gills?

Because they are not actually nostrils nor used for breathing.

The two holes on the front of the shark are used for sensing smells, but not for breathing.

While the shark swims, water flows through the "nostrils" and along the nasal passage. Inside the nasal passage are folds of skin which are covered in very sensitve sensory cells. These cells enable to shark to "smell" things. For example, a shark can "smell" blood through the water passing into its nostrils, pinpoint the source of the blood and hunt the prey.

So, the nostrils are used for "smelling", not breathing.