[MBCook]

Basically a values thing that they are dedicated to military personnel and their families. Some of their products are open to everyone (IIRC), but most are limited to military, spouses, and immediate family.

From Wikipedia:

USAA was founded in 1922 by a group of U.S. Army officers to self-insure one another when they were unable to secure auto insurance because of the perception that they, as military officers, were a high-risk group.