Selecting lots of elements on the map might result in a large number of map markers. This can result in a difficult to read map. By turning on the cluster option, your map will always be clean and easy to read.
High-density marker areas will automatically be grouped and color coded. Hovering over such cluster will reveal the area it is covering.
Clicking on a cluster will automatically zoom into the selected area. As the density decreases more markers will show up. See the before and after picture. Note the blue polygon containing 47 elements.