By releasing the e-mails, Trump Jr. undermined the ability of the media to make blatantly false representations in their reporting on what was in them.
Go back to the whole James Comey charade and see how easy it was for the New York Times, Washington Post, and CNN to run stories with claims about the contents of Comey's memos that turned out to be completely false after Comey's testimony was made public.
The press is still running hard with whatever investigations are on Mueller's plate and in the Congressional Committees. "We can't tell you what's in it" is turned into "something bad" by the work of pure speculation. Comey had this down to an art, in fact, he's personally responsible for seeding the field so it could sprout baseless incrimination against Trump.