Grab Passes are a really complicated and expensive way to do nothing in a shader. A Grab Pass is an additional pass added to your shader that captures the scene and allows you to use that captured texture in your shader. Want to make refraction’s in glass or an energy shield? This is a good way to do that.
Unity has examples on how to do this using a vertex and fragment shader. We’re going to explore how to do this using a vertex and surface shader.
More info about Unity’s Grab Pass:
Some discussions from the Unity Forum about using the Unity Grab Pass in a Surface Shader: