The patient has to get exposed and positioned, then padded (so there are no pressure injuries, no errant cables or equipment pushing on skin, etc). Also under anesthesia (depending on the type but I’ll assume general/completely asleep) you aren’t moving and your body may get moved or shifted into an unnatural position.
It’s also nice to have controls as mentioned by another reply, but pulse oximetry is great, and can be slapped on any non sterilized area to assess oxygenation.