Custom shapes are essentially exactly the same as other shape tools such as square and ellipse. this feature wont be used very often for photo restoration. The beauty of the custom shape tool is that you can design your own shape using over tools such as the pen tool or brushes and then save the shape as a custom shape. then all you have to do is load in this new shape and you can drag it out on a layer. As with the other shapes, if you hold down the shift key while you drag it out the proportions are constain. As you can image this tool is hardly ever used for photo restoration. one example of where it might be used for cosmetic enhancements would be if you had a custom shape that was the same shape as an eyebrow and you used the shape to create a selection path and then used this a path with the clone stamp tool to crete the perfectly shaped eyebrow.