It gets oxygen in and carbon dioxide out, when you aren't active. It's sort of a substitute for heavy breathing, like you do when very active and your cells need oxygen faster.
So if you are a comedian and the crowd starts yawning a lot, I suppose you can joke about it being better than heavy breathing. I'm no comic, so I'm sure you can think of something better.
Why do people yawn when bored? They really are just breathing shallowly and don't realize it until boredom makes them aware that they feel tired from lack of oxygen.

Please tell me why you posted this in 'Humor and Comedy?' Is it intended to be funny or are you asking a serious question please?