What does a fracture walking boot do?

I recently got a comment from Michael on a video on the Doc On The Run YouTube channel which was about how fracture walking boots are one of the worst treatments for an injured runner. 

He posted an interesting comment which is really a valid question. He said, 

“What is the boot for? Is it for protection? Is it for immobilization? Is it to keep weight off?"

What does a fracture walking boot actually do? 

Well, that's what we're talking about today on the Doc On The Run Podcast.