Direct line to the head of the agency, who is in a position to help advance the goals of the members?

Without the direct line, the association has to rely on lobbying/bribing politicians for access.

An advisory counsel may not get paid but they submit findings to the Secretary. There's a long tail where the government becomes more aware of long standing and emergent issues.

Industry associations have no such reach unless individual members make it so, and unofficially at that.