I let a user pick their route to work...The get on #80 going east bound at Cicero. Then take the Ashland bus going South.
I'm riding the #80 Irving Park bus eastbound and I want to know how long its going to take for my bus to get to Ashland. If I know the lat/long for an Ashland/Irving Park stop, I could just supply that rather than trying to determine the #80's intersecting stopid.
The API could take the supplied coord then determine the route's closet coord to the point, then return that ETA. This could be immensely helpful and simplify a pretty handy use case.