Grouping layers can be really handy, for example, if want to move multiple items at once.
Grouping and ungrouping layers
Hold down the SHIFT key to select the objects in the Layer Manager. Group or ungroup layers in one of the following ways.
You can group and ungroup objects directly on the map. For more information, see Work with Lines and Shapes.
You can nest layers within layers by dragging the layer and dropping it below the destination layer. You'll see a blue underline indicating that this is the parent layer. You cannot nest lines and shapes within similar objects. If you want to nest an object, you must add it to a layer.