Purpose of maxExtent?

Question

I am starting with maps but I am a beginner.

I cannot figure out why to use maxExtent. If I delete it from my application, its behavior doesn't change or I do not notice.

So why should I use it?

I understand purpose of restrictedExtent but not of maxExtent.

var extent = new OpenLayers.Bounds(-2003750.34, -2003750.34, 2003750.34, 2003750.34);
    
map = new OpenLayers.Map("map", {
        controls: [
                        new OpenLayers.Control.Navigation(),
                        new OpenLayers.Control.PanZoomBar(),
                        new OpenLayers.Control.ScaleLine(),
                        new OpenLayers.Control.Attribution(),
                        new OpenLayers.Control.LayerSwitcher()],
        maxExtent: extent,
        //restrictedExtent: extent,
        maxResolution: 156543.0399,
        numZoomLevels: 19,
        units: 'm',
        projection: new OpenLayers.Projection("EPSG:900913"),
        displayProjection: new OpenLayers.Projection("EPSG:4326")
    });

var osm = new OpenLayers.Layer.OSM.Mapnik("Mapnik", { 'attribution': 'Provided by blop' });
var gmap = new OpenLayers.Layer.Google("Google Streets", { visibility: false });
     
map.addLayers([osm, gmap]);

Answer 1

maxExtent is used for other projections.

For instance by default it's the whole world in decimal degrees (-180, -90, 180, 90)

But if you are using a different projection such as EPSG:3857 You will need to modify the maxExtent. One that is valid for that projection. Which in the case of EPSG:3857 could be (-20037508.34, -20037508.34, 20037508.34, 20037508.34)