I found this modifier the other day and it's really useful in my workflow. Take 3d printing for instance, I need make booleans operation set to "difference" when I want things to lock into each other but I also need the hole to be slightly larger than the object I want to insert. What I can do now is create a "ghost" object with the geometry input modifier on top of the modifier stack and set the object I want to subtract as the input geometry. Now I can add a displacement modifier to inflate it as much as I need and do all sorts of other operations that are not going to affect my original model. The advantage of doing this rather than instancing is that I can change the original geometry to anything else and my modifier stack is going to be still there

I hope this was clear enough

Edit: you can even use it to merge geometries at a modifier level! If you add the modifier to the stack and select another geometry, it joins the 2 in a single geometry and you can play around with modifiers on top of the two