"Drone" gets used to cover a lot of things; full aircraft sized Reaper/Predator drones down to toy-sized quadcopters. It's the latter which Ukraine has been developing, including a unique solution to ECM: the fiber-optic drone.

Small drones do not assume the role of tanks. Drones assume the role of WW1 aircraft: artillery spotters and very light bombing capability. They have this role there because both sides have SAM superiority over the other's airforce.

Drones solve the problem that combat aircraft are too expensive and too easy to shoot down.