The official public reason for NASA having an SR-71 back when they were still flying one was for extreme high altitude atmospheric tests/experiments. No other plane (at least at the time) could fly at the altitudes the SR-71 could.

Whether that was a cover story for additional spying by the NASA SR-71 is unknown to those of us without the necessary security clearances.

For visual and SAR, sure, but there are other missions like electronic warfare and signals intelligence that are still well served by aircraft.