Grouped elements appear as a single object. Selection and changes always apply to all elements of the group. Of course grouped objects can again be grouped at a higher level. The reverse process works too. Layers First introduced in expensive CAD tools, layers are now a standard feature for all major drawing applications.
Layers are another level in the hierarchy, similar to the grouping feature.
Objects on a layer however are still manipulated as distinct objects. You can turn off display or printing for whole layers, move all objects in the layer up and down in the Z order and enable or disable editing. Layers are easily accessible through a docker window. Very often you will want to add the same effect to many objects. Often you will find yourself doing the same job over and over again.