Why it's useful:
Large images - allows quickly viewing at full resolution and fitting the screen resolution without the need to scroll.
Small images - allows zooming in quickly to fit the screen and returning to actual size
Other reasons it's a useful feature:
When right hand is on the arrow keys (navigating images) and left hand is on the left side of the keyboard, scrolling to zoom in and out requires moving a hand back and forth. For quick easy navigation the space bar could be used for cycling between full size and fit.
Could be an unmapped hotkey by default.
Why it's useful:
Large images - allows quickly viewing at full resolution and fitting the screen resolution without the need to scroll.
Small images - allows zooming in quickly to fit the screen and returning to actual size
Other reasons it's a useful feature:
When right hand is on the arrow keys (navigating images) and left hand is on the left side of the keyboard, scrolling to zoom in and out requires moving a hand back and forth. For quick easy navigation the space bar could be used for cycling between full size and fit.
Could be an unmapped hotkey by default.