People are talking elsewhere in the thread about glasses fogging up. That, even with an extremely non-airtight mask, that is reused, touched constantly, etc. seems like prima facie evidence to me that it deflects the stream of droplet-laden air from your face, thus inhibiting it from spraying on to others.
Like others have said, if this were a different disease, maybe it would be spread more by contact and masks wouldn't be worth bothering.
And if you're only focused on how a mask can protect you, maybe it doesn't help that much. But if you are infected without knowing it...
Edit: my pun was unintentional...really, I swear.