Thanks for your question. You can make reference to my latest work on “Identity in Hyperledger Fabric” (link) and see if you grab some idea on both cryptogen and Fabric-CA.
In short, cryptogen can generate the required certificates for the peers and users specified in the crypto-config.yaml. In the First Network example, as it’s more for demo, we do not need to generate additional identities. Therefore we don’t need Fabric-CA.
If you need additional identities (e.g. more users wish to access the fabric network) you can generate (enrol) them on Fabric-CA, which has the CA information created with cryptogen. FabCar (link) is a good example, in which additional identities are generated to perform the client applications.