Suppose the U.S. is testing a new secret fighter jet. If you don't know when the new jet will leave the hangar for a test flight, your satellites will be useless. Long-distance photography would perhaps provide some intel. Of course, if the test flights were carried out at night, as it happened while the F-117 was being tested, then photography would not help much,

Do note that when most USAF bases in the Southwest were built, there were no satellites at all, and hence, no spy satellites.