They do spying to understand the capabilities of allies and adversary weapons. The goal is to maintain supremacy and slow down development. The US had a spy in the Soviet era that provided the entire Soviet radar roadmap into the 1990s, and technology was deployed (or not) based on that.

That’s one of the reasons drones are so disruptive. The development cycle is tuned by state of the art missile and stuff, and isn’t tooled for some dude strapping a mortar round to a little drone.