This is a combined pan and zoom tool. To use it: Activate the tool. Then LMB drag a rectangle on a view window. Note how the dragged rectangle has automatically the proportions of the view window. After left-to-right dragging, the defined rectangle area is magnified to fill the entire view. Reversed dragging from right to left will shrink the current view area to fit the defined rectangle.
To just pan, without zooming, just click the mouse button once, without dragging.