For example, think about if you shot a photo with an ND grad, and the subject is sitting above the horizon, like a lighthouse. The ND effect would apply to both the sky and the lighthouse.
If you shot this photo without a filter, people would mask the lighthouse when applying a gradient filter, thus leaving the lighthouse untouched while editing the sky.
So what do people generally do when you use real ND grads, do you go back and try to undo the effect it has on the lighthouse? Or would you leave it alone?