I'm not sure if I understand your question correctly, but a simple example comes to mind: Let's say you have layer A with an opacity of 100%, then layer B with an opacity of 50%, then layer C with an opacity of 30% (and so on). You can't combine these layers because they have different opacity settings (or blending options or effects or tracing or whatever).
Let's say all these layers were for a bird in a tree with lots of leaves and details. You'd certainly want to have that bird isolated in a group to organize your work and not have to sift through thousands of layers. Furthermore, let's say you have several birds, so you decide to create a group that contains all the groups of birds, and then create a group that contains all the groups that contain the layer for the tree and the birds, since this is part of a larger illustration.
In short, what I'm saying with this example is that the two are conceptually different. Layers are the layers of content (sorry for the redundancy) that make up the artwork, that is, an artistic tool. And yes, you can arrange them any way you want, but if you arrange them in different ways, the artwork changes.
On the other hand, grouping is an organizational tool that is mainly used to create order. Combining layers is likely to cause a lot of problems (unless it's intentional), while combining groups is unlikely to do anything (unless the stacking order is wrong, but you can change that)