They are necessary for AI-Ship as an approach-path.
Example :
When you order an AI-Fighter to attack a turret or subsystem without a path added to it, the fighter will start firing at it from right were he stands (if he's not forced to evade some AA-Fire). He might even fire on the underside of a ship in order to destroy a turret on the top-side (of course with not much effect).
But when you set a path to a turret, then the AI-Fighter will fly to the end of the given path and start his attack-run from there.
It's the same like the hangar-paths, there are just there to control the flightpath of AI-Ships.
The only thing I never really understood is, why you can specify turrets to some AI-Paths, maybe something like a "this path is guarded by these turrets", though I don't really see any reason in this.