Fat tree network topology looks like a tree topology like below figure. In tree topology, we have same terminologies like Root, parent, child etc. This is mainly used to connect a large number of physical servers/computers in a large data center.
[![enter image description here](https://i.stack.imgur.com/tkmdh.png)](https://i.stack.imgur.com/tkmdh.png)

In the tree structure topology, leaf nodes are physical servers or computers. Rest other nodes are switches. Switches are basically 3 types: Core switches, Aggregation switches, and Edge switches. My question is what's the purpose of such a structure, and i want to know the functionality of each layer (ex: which layer has a routing role ...).