to prevent bodily fluids from splashing into their mouth.
"The use of masks, impermeable surgical drapes, hospital-laundered surgical scrubs, and modern surgical gowns—all staples of infection control in the operating room—were not guided by clinical studies [14]. As with many practices in medicine, behaviors passed down from one generation to the next become common practice and are subsequently mandated by governing bodies [15, 16].
In fact, the argument for many of these practices is as much about protecting healthcare workers from their patients as it is protecting patients from healthcare workers’ germs"
https://academic.oup.com/cid/article/65/9/1589/3858163