To reduce the size of an image or remove non-important aspects of an image. For example, you may have a picture of several people, but may only want to have a picture of one person from that picture. Making a selection around that person and then using the crop feature makes a picture of only that person.
Another common mistake often seen on the Internet is users who scan a picture using a flatbed scanner and have a large blank area around the image. Cropping an image reduces the overall size of the image and decreases the time it takes to download. The picture is an example of what a non-cropped and cropped image. As can be seen, the original image has lots of wasted space making the image visually and physically larger. Cropping the image reduces the overall size of the picture and gets rid of wasted white space.