OpenStack Networking shouldn't be considered a standalone product item that a developer might consume, rather, it's one piece of a larger platform that one consumes when deploying virtual machine instances. If you're developing an application, whether it be hosted on a single VM, in a traditional 3-tier design, or across containers on multiple VMs, you will need to understand the capabilities of OpenStack Networking and related API so that you can build out the (virtual) networks, subnets, routers, ACLs, etc. accordingly to support your application.
The book provides information to the cloud architect/operator on how to properly install OpenStack and related networking components, and also provides information to the cloud user/application developer on how to interact with the API and build the virtual network objects. As a user, you may be presented with pre-existing networks setup by the operator and never need to venture any further. Other users may be required to setup one or more virtual routers, networks, ports, etc. depending on the needs of their application and/or virtual machine instance(s), as the one-size-fits-all approach may not be appropriate for their use case.