The Discoverer program was in reality a cover for the CIA's Corona program...
The Corona satellites took photographs of "denied access" areas in the former Soviet Union. The film was then included in drop cans that were to be snatched from the air by the recovery aircraft.
Many of these canisters were lost, thus endangering the secrecy of Corona. I imagine that the Agency was really delighted when the technology for transmitting pictures electronically was developed.