Primarily, any license fees your don't have to pay for billions of per-use occurrences is money saved. In Twitter's case: that's a lot of money.
But there's of course also the whole "Your honor, as the evidence makes plain, this content/communication is claimed to have originated on Twitter, but does not use our official typeface. We move to dismiss this case/be awarded damages based on the fact that we were not part of this particular incident", paired with "Your honor, as the evidence makes plain, this content uses our proprietary font, but did not originate at twitter. We move to dismiss this case because we were not involved/be awarded damages on the basis us defamation and will be filing a counter-suit for impersonation".
There was that incident a while back where a bunch of government leaks were outed as fake because they were printed with a (default MS Word) font that was not available or in use when the document was purportedly created
Then Pakistani Prime Minister Nawaz Sharif tried to cover up a scandal that was revealed in the Panama papers back in 2014. He forged a document purportedly from 2006 using Microsoft's not-yet-released font Calibri as an attempted cover story.
