The question is "why do we yawn", not "what is a yawn". This asinine paragraph is talking about why we open our mouths to yawn? WTF, that's what yawning IS.

And it also incorrectly asserts that yawns are associated with a deep breath. You hear this all the time, but a yawn does NOT induce a deep breath; they may sometimes happen at the same time, or perhaps people simply feel a yawn is also a good time to take a sighing breath. But a yawn is limited to the musculature of the jaw, and that's it. Try it. You do not need a take a breath when yawning; it has nothing to do with it.

I am none the wiser as to why we open our mouths to yawn properly, or why we yawn at all. Describing the consequences or side-effects of yawning does not tell us why we do it. There are various theories to be found on the subject (to increase the oxygen level in the blood, the redistribution of the alveolar surfactant, etc) but nobody, including Dr Juan, seems to know. Why, then, even bother asking the question?

A real cargo of redundant aged shoemakers

"The involuntary act of yawning usually includes opening the mouth very wide while slowly taking in a deep breath"

Back to the original question, why?

The article seems to be missing a step. What causes us to open our mouths in the first place?