This is for people for whom full membership in the party isn't the right fit. Maybe that's a matter of capacity or maybe it's a political disagreement that would make adhering to all of the party's positions difficult or uncomfortable. Or maybe you're in an area with no party presence and can't take on the very high-effort task of building a branch. Whatever it is, this is a way to build a consistent relationship with the that includes organizing, education, and financial support for the party's work.
Throughout every historical revolution, the vanguard party, even when it becomes a mass party, is never a majority of the oppressed classes. It isn't intended to be, and many of the people who play a role in the revolutionary movement will exist on the periphery of the party, supporting its work without joining.