Have you ever tried to precisely crop an image when there are just a few pixels that you want to get rid of? The crop tool just snaps to the document border or to the layer border. Sometimes I just zoom in to make the difference much larger and then adjust the crop area.
But there is a better way - Hold Ctrl key (or Command on Mac) and adjust the crop area to precisely crop your image - down to a pixel.
As well when you use your crop tool you can easily move the crop area with the arrow keys and zoom in/out with Ctrl/Command "+/-" keys.