When you crop an image, you essentially change the pixel size. Resizing an image can make the image clearer or even make the file size smaller. When you resize an image, you don't cut out any parts of the image, you simply change the aspect ratio of it. Cropping an image means you cut out parts of it. Whether you want to change the focal point, or you want to get rid of something in the background, this is what cropping is for.