The most obvious reason to crop a photograph is to remove distracting elements within the image. For this first image, I wanted to isolate the woman for a nice clean portrait photograph.
Using the Rectangular Selection Tool, I made a selection around the figure. When I was happy with the selection, I selected Image > Crop Selection, and I was left with just my cropped figure. By doing this, I eliminated the excess background and all focus in the picture is directed on the woman now.