One example would be if you double clip on a clip to select it & you wished to just zoom in from that view that occupies the entire screen down to something that rests against the bottom edge. To do this I just move the selection handle to make my selection zoom in. This zoom maintains the pictures original ratio. The new selection is now smaller but its located in the center of the original.
Because I want the bottom of my selection to touch the bottom edge of the frame I still have to drag the entire smaller selection down so that it touches the bottom. Once its touching the bottom edge I still might have to re size the selection to get the cropping correct. If the selection tool had allowed my new selection to snap to to any selected edge this whole process would have been easier. I hope that you can understand just what I’m about.
Thanks in advance