Big picture, they protect the patient from someone accidentally coughing up large droplets and/or gum into the open wound. Masks offer little protection to OR staff, especially when considering use of electrical cautery (to stop bleeding) often aerosolizes particles of who knows what.
Read a study done about 20+ years ago in British ORs. Only those immediately next to the OR table wore a mask. Those further away such as the circulating nurse, anesthesia, and anyone else that came into the room didn’t wear a mask. Results, no increase in patient infection rate.
If you want to wear a mask, go ahead. Higher rate of return on investment is washing your hands and not touching your face.