Well Z lets you sculpt lots of detials directly on the object in a way that you cant in photoshop or maya. You can then export the maps directly from the model etc etc.

You could do it in maya and photoshop, but you get more control over the displacements with Zbrush.

Its not to say you cant do great work without Zbrush, its just another tool to have. But you need to work at it to get the detials in there.