What kicks off a watchOS background URL download

I'm trying to understand the background URL download from the "Keep Your Complications Up To Date" video. I'm confused as to what the refresh method does. In his example, all it does is stores a completion handler.

If schedule(first:) is what actually schedules/runs the file download, why not just pass/store the completion handler there?

Seems like my applicationDidFinishLaunching method would call schedule(first: true)?