Probably so they could keep an eye on the toenails on the non-operating foot.
There’s a reason they tell you not to wear nail polish before surgery. The nailbeds are one of the best ways to detect cyanosis caused by low oxygen levels in blood.
I’d imagine a “control foot” is probably preferential, and it’s easier to keep an eye on the other foot during surgery than it is to keep an eye on their fingernails.