I thought the idea was that back when we were in settlements, keeping watch out into the dark, the yawn would happen to a sentry if nothing was going on and would subconsciously signal to other humans, even the watch dogs that " nothing is going on, all is well" then spreading throughout the camp as sort of a psychic link, literally conveying information at the speed of light to anyone who can see you.

At least that's what some article I read had to say about it

I’d say for empathy and humans showing connection by yawning when others do.