What Regardie published was the working material: the initiation ceremonies of the grades from Neophyte upward, the Knowledge Lectures the members had to learn for each, the rituals of the pentagram and the hexagram, the consecration of the implements, the system of skrying, the Enochian material, and the tables of correspondence underlying all of it.
The scale of the disclosure is what made it consequential. Before 1937 the Golden Dawn system existed in oath bound manuscripts held by a handful of people in fragmentary and mutually inconsistent copies; after 1940 it existed in four printed volumes that anybody could buy. Effectively every Western ceremonial order founded since works from this text or from something derived from it, and a great many practitioners who have never belonged to any order use its rituals.
Regardie's stated reason was preservation. The order had fractured, its surviving members were elderly, and he judged that the system would be lost. His critics judged that he had broken an oath, and both things are true.
The atlas records the publication as the decisive event in the transmission of Western ceremonial magic, and describes the practice rather than instructing in it.