Harris corner detection shape of $E(u,v)$
$E(u,v)$ is zero for zero shift ($u=v=0$), where the two patches being compared are equal. As you increase the distance, the difference tends to increase. Depending on the local shape of the image, the $E(u,v)$ surface will have a different shape: at a corner or a small point, it will steeply increase from 0 in all directions. Along an edge, it will remain close to 0 along one direction, and increase steeply in the perpendicular direction. At flat image regions, the function will remain closer to zero in all directions.
The Harris corner detector (and other corner detectors too) look at the shape of this patch, if it increases steeply in all directions, the detector will produce a stronger response.