Thank you for mentioning this!
A guy by the name of Dave Straker did a whole book about PostIts and problem solving. When I was working at a certain mega-corp's HQ, they were using this combination. Straker's book came out in 1997. I was working with people who routinely used the whiteboard/PostIt combo as early as 1988 - so who knows where it first originated.
And who cares, really?
Well, it's kinda intuitive. White/Black boards, Paper - call them cousins. (On paper, years ago I made a system of pencil and blue pen - pencil can be erased with some effort; the blue pen is "bold" if you really know what you are doing, but it becomes a bad mess if you mess it up too many times!)
If you have a genius layout and do it right, you get a beautiful exhibit, paper lasting longer than the boards. But if you are still thrashing about, and you start getting the layout wrong, it might look cool all "mad scientist", but also definitely crowded and alpha/beta and not ready for production. (Mad scientists always put beta stuff into production! Hehe!)
So if you half draw stuff on a board, and half on stickies that you move around, it makes just fine sense.