The purpose of this model is to clarify the fact that each layer exists on different kinds of network, and that (within reason) the different kinds of network can be mixed and matched.
To take some practical examples of this, an Ethernet (a kind of physical network) can carry many different kinds of networking protocol: Internet (IP), Novell, Appletalk, DECNET etc. and furthermore, a given type of networking protocol can be carried over different kinds of physical network (IP for example can be carried over Ethernet, point-to-point links, even over radio). To add even more complication, some networking protocols can be encapsulated in other protocols, with the encapsulation being stripped off elsewhere. The most complex situation that we encounter is the Particle Physics DECNET network, that is carried over an X25 network, which in turn is carried over the IP network!!
That was a whirlwind tour of the layer cake model, and not particularly well explained I fear, but I haven't time for a full exposition. Suffice it to say that within the Department, we make use of the following combinations: