I used to use this functionality in another software. I was very useful especially with large assemblies as well as drawings. There was a keystroke that invoked this and what would happen is while you are zoomed out, a magnifying glass would appear that as you moved it over your model, would allow you to zoom in tight and pan within this magnifying glass without affecting the zoom factor of the master view.
The whole point was to prevent the constant zooming in and out of large assembly models and drawings.
I hope this makes sense.