Why use bone as target for IK? I have seen numerous tutorials about rigging and animation of armatures. Whats bugs me is why everyone of it uses a extra bone as IK target and not something that is not related to the armature? like a spehre or other primitive mesh that is not part of the mesh?

Imagine the case where I don't want to render the animation in blender but want to export it then  there would be two useless bones along with the Armature which are not used in animation anymore.

Just why use a extra bone why not animate with a seperate object or mesh for IK?
The real reason is Bone targets positions can be stored in pose library. Also the scene is cleaner if everything is under armature.

But if you want to export the armature and another rig setup suits that better its ok. You can use anything you want for IK target.