In many of the JS add-ins that I was asked to build, users needed to interact with basic shapes (arrows, rects, ovals), change their properties (colors, pen style, fill), change their positions and sizes and virtually do a lot of stuff that is available in the UI. These actions can be done using VBA and VSTO too.
Same goes for non-inline pictures. One of my JS add-ins needs to manipulate picture positions and sizes and access the "Compress Picture" functionality.
Last but not the least, the same add-in needs to get the position of an INLINE image relative to the page. I'm not sure if that makes sense because this can't even be done in VSTO or VBA (obviously because it's an inline element), but I'm still looking for a possible way of achieving this, because the add-in needs to inject a Rectangle shape at the center of selected INLINE image.