From Jeff Mason, WHCA president:
"White House briefings and press conferences provide substantive and symbolic opportunities for journalists to pose questions to officials at the highest levels of the U.S. government. That exercise, conducted in full view of our republic's citizens, is clearly in line with the spirit of the First Amendment. Doing away with briefings would reduce accountability, transparency, and the opportunity for Americans to see that, in the U.S. system, no political figure is above being questioned. The White House Correspondents' Association would object to any move that would threaten those constitutionally-protected principles."
Background: This series of Trump tweets.