Peacocks with their giant tail feathers are my favorite example. They make flying really difficult, but they make attracting female mates much easier. The reproduction need wins.
And that, my friends, is why I bought a Pontiac...
I assumed it was a kind of warning system: "Another member of the tribe detected impairment/fatigue and took measures to become more alert, perhaps you should raise alertness also."
Interesting you went in the opposite direction of my assumption, which was "another member is tired, perhaps we should all pack it it"
AFAIK yawning is something our bodies do to fight or delay sleep, rather than to get to sleep faster.