[Mrs.K]

Is there some unknown purpose for this sudden and seemingly meaningless-with-no-reason burst of energy?

The triggers seem to be many, at least for Wally:

-I'm wet
-I'm in the rain
-I'm wet and in the rain
-It's windy and it's blowing in my face
-There's a lot of open ground here!
-You threw the ball and I'm just gonna run around!
-I haven't done this in a while
-We just finished a training/shaping session and now he's running everywhere in joy (I guess?)

So, I ask you all who are way smarter about dogs than this 1-dog, first-time owner. What IS the purpose of zoomies?

You forgot the most important reasons of all: "I'm a happy dog!"
That's why they do zoomies. It's an expression of enjoying themselves being perfectly okay with themselves, having fun, energy and loving life.

Question for you: WHY NOT ZOOMIES?

Zoomies are fun. I loved the Zoomie stage.