Ah. In that case, I seem to recall they performed user research among users of their browsers that uncovered that sending files to others was still a major pain point. It's also a way in to promote a Firefox account, and Firefox in general.
Of course Mozilla's not in it for the money, so there's not a direct line from Send to more revenue. Firefox is their main tool to protect the open web, and Send is a way to get more people to use that. And of course, being able to send files encrypted is good for the web as well.
Indirectly, it is primarily financed by the search engine deal in Firefox.
