Shields.
Put a swarm of a few hundred those things around a naval vessel, each carrying a slab of metal plating beneath them. When you detect a projectile or a missile approaching the ship, the drones move into a screening formation and the projectile hits one of the plates, detonating at a good standoff distance and doing no damage.
On a smaller scale, a few dozen of these things around a tank or an armored vehicle would make a pretty effective screen against RPGs or mortar shells.
I guess that would work if they were cheap enough to be replaced easily.
As for surveillance, there's speculation that governments have already deployed roughly dragonfly-sized mobile cameras. These things might be more maneuverable, though. If they can be made to run dead silent, it would be hard to notice them
and they could get into really tight spaces.