If you have an image that contains one or more layers that extend beyond the visible area of the image you are working on, parts of these layers may become visible in a newly cropped image if Allow growing is selected.
You can see in the image how a selection for cropping has been drawn that extends beyond the boundary of the image. The small image below it is the resulting cropped image. You can see that because the layer extended beyond the edge of the image, Allow growing has resulted in a new photo with the lady walking the dogs now in the center of the picture.