According to the website, the Baseball Writers’ Association of America was founded in 1908, and was established to:
Assist journalists covering Major League Baseball for daily newspapers. Its purpose is to ensure proper working conditions in press boxes and clubhouses, and to ensure its members have access to players and others in the game so members’ reporting can be accurate, fair and complete.
Today there are more than 700 active members of the BBWAA working for newspapers, magazines and major web sites. The main requirement for membership is still that a writer works for a newspaper or news outlet that covers major league baseball on a regular basis.