Right now, drones don't have an ability to follow units, there is no follow waypoint in the editor either, I guess because it's possible to achieve the same functionality by grouping units together, but since drones don't use AI, you can't order them to follow you or a specific vehicle.

I can see many uses for a follow functionality for drones, for example:
-You have a car/squad that you want to follow, perhaps to protect them and support them, so you assign a quadrotor to follow them, or perhaps they're an enemy unit you want to gather information on and maybe destroy, so you assign a high altitude UAV to follow them, so they're always in his sights. (Unless somehow it loses track of them.)

-You want to destroy an enemy tank, you have an approximate location of him obtained from some ground unit but it's not exact. If you'd place a 'Destroy' marker there, it might attack another vehicle nearby that you don't want to destroy, such as an MRAP. So if you set your drone to destroy the tank specifically, it will go to its last known position and if he finds out the exact location of the tank, he will try to destroy it.

So, an idea of implementing this feature:
You can double click on an object in the map(infantry unit, vehicle) and this way, the drone will do a specific action depending on the type of the waypoint, on the targeted unit.

For example, you double click a tank and set the type to "Move", the drone simply follows that unit.
Or, you set the type to loiter, the drone circles around the target, and updates the loiter waypoint when the target has gone more than 75% further from the loiter radius center.
Or, you set the type to destroy, the drone looks exactly for the object that you doubleclicked on, whether it's a vehicle, soldier or even a stationary object. Once it finds it in realtime, it will target that unit/object and lase it/launch a missile at it.