The answer for the question here would be that it provides access to the rest of the network through switching, routing, and network access policies. The three layer heirarchial model that is shown is a very good example of using a graphic model in order to show how things work.
This is a very good way of showing exactly how access is being provided across a network. The three layer model allows for more complex manuvering, designing, and manipulating of any computer network. Of the three layer heirarchial model, each layer is responsible for a different part of the operating network.
There is a distribution layer that provides support to the other two layers which are the access layer and the core layer. It is the distribution layer where things such as routing and access to the wireless access network or better known as WAN. The distribution layer has mainly the routers and the switches of the computer located there.