They act as a baffle so besides redirecting they also rob a bit of kinetic energy but there's no reason to think either of these effects actually help. They probably also stop the very largest drops which have too much momentum to be redirected but as far as I know they still don't know what size drop are most concerning.

Really though if masks worked then masks would work and we would know they work without some data scientist running a model that shows "oh if we didn't wear masks the cases would be one percent higher." I was promask early on because "maybe masks work" but then everyone started wearing masks and nothing changed.