That Leaflet plugin allows you to use a DIV as the active area. All positionning methods (setView, fitBounds, setZoom) will be applyed on this DIV instead of the all map. A typical use case is to set a map as background (like that example) and center it on the top of the screen for example. Defining media queries on that DIV make it easy to adapt the active area according client resolution. Then, the map will center itself in the center of that DIV when calling setView, setZoom, fitBounds, etc.