Question:what is the benefit of using layer set? 

Layer sets are folders into which we can collect several individual layers. The benefit of using layer set  is that by using layer set we can turn on and off all the layers grouped in a layer set at a time. We can also move them all at once with the Move tool, and resize them and reshape them all with the Free Transform command.

Question:How can we use the Clone Stamp tool in Adobe Photoshop for retouching/ repairing  an image. 

The Clone Stamp tool is used to paint one part of an image over another part of the same image or over another part of any open document that has the same color mode.So, we can use the Clone Stamp tool for retouching and repairing a defective image.