Review (4/5): Not a great fit on my face (long, narrow nose), my my daughter really likes them
Providing you have a mask that's roomy enough to accommodate the frame, these effectively lift the fabric away from your nose and mouth, making a mask feel less oppressive, especially when you're breathing heavily (exercising, going up and down stairs a lot, etc.).
I have a long, narrow nose, so fit is not great on me, but it works well for my daughter.
If your mask is not big enough, this make make it gape, ruining the effectiveness of wearing a mask, so check the fit with it before you go out.