It’s not merely advisory. It has a real legal authority which can result in your getting a ticket - and that would be for people who left the footpath after it has started flashing*. Essentially, it has a very similar meaning to a yellow light for cars - do not enter the intersection unless it is too late for you not to. I suppose it could also be advisory in the sense of “oh shit, I’m walking too slowly, I’d better hurry up”, but I know I don’t pay attention to that light after I’ve stepped off the footpath, and I suspect most other able-bodied folk don’t either.
*In Sydney, there used to be small enamel signs explicitly stating this:
“WALK - you may cross the road
Flashing DON’T WALK - you may not start crossing the road, but may complete your crossing if already doing so.
Solid DON’T WALK - you may not cross the road.”
These signs seem to have disappeared, but they were everywhere when I was a kid.