When working with complex rigs, you may want to group layers together to avoid cluttering the Timeline view and keep your layers organized.
When grouping layers, you can choose to create a simple group, or a group with a composite. A simple group will simply help you organize your Timeline view, whereas a group with a composite will connect all the nodes in your group to the same composite, allowing you to apply the same effect to all of its layers at once. For example, if transparency is applied to each individual layer making a character rig, the seams between its body parts will become visible. Instead, if you put all of the rig's layers together into a group with composite, then add a Transparency effect to the group, Harmony will compose the layers inside the group into a single image before applying the effect.