When working on Artboards with large numbers of groups and sub-groups, it’s easy to get lost and waste time scrolling up and down. Manually collapsing each group is also a waste of time.
Why not add a feature to collapse all groups in a single Artboard with just one click?
Just a note that on that specific shortcut that @sonburn and @ck01 used: ⇧⌘↑ is the default Sketch shortcut for reducing the height of selected layers by the nudge amount, so if you set that as a custom shortcut for something else, you’ll be unable to do that default action with the keyboard.