This is something I learned very recently. Using a polarizing filter in studio can help reduce the glare on people's skin. Not everyone comes to a shoot with makeup, especially men. If, like me, they have an oily skin, the light can reflect on it. The polarizer can help reduce the retouching work (we always like that).

The left photo was taken with a polarizer. You can see the glare on the forehead is more pronounced on the second picture. It is hard to adjust the polarizing filter though. When you shoot outside, it's easy to see the difference in the sky or on water. For the studio, it's more guess work and trial and error. I think it's worth the few minutes it takes to set up though.

Using a polarizer (left) in studio to reduce the glare on the skin (right).