Hey, that's a neat tool!
I think it's really helpful to help visualize the invest vs/ pay down mortgage question. "If I could put $X extra towards principal per month, I'd pay off my mortgage Y years early... what would have happened if I invest that 100/mo instead?" Gives you not only the average likely return over that time span, but probability (i.e. "a 9% chance I will have a negative return on the money" or "1/3 chance I will return less than my 4% mortgage, so 2/3 chance I will make more money investing", etc.).
There's a few other uses that immediately spring to mind, but that's a big one.
I like it!