Renderlayers are used to separate your composite image into layers. Use RenderLayers for a specific reason - such as creating depth of field, relighting isolated elements within the image via a normal pass, adding a colorcast to specific portions of the image, etc. The keyword here is isolation. Renderlayers allow you to dissect, effect and or correct individual elements or groups within your composition before outputting your final render. This saves you from endlessly re-rendering your scene just to find out whether a correction is going to work or not.