While it is a little dated, this is a wonderful site for those looking for specific information on number stations. Their purpose is known, and it is primarily for clandestine operations.
"several articles in the radio magazine Popular Communications published in the 1980s and early 1990s described hobbyists using portable radio direction-finding equipment to locate numbers stations in Florida and in the Warrenton, Virginia, areas of the United States. From the outside, they spotted the station's antenna inside a military facility. The station hunter speculated that the antenna's transmitter at the facility was connected by a telephone wire pair to a source of spoken numbers in the Washington, D.C., area. The author said the Federal Communications Commission would not comment on public inquiries about American territory numbers stations."