Let's first answer the question, what exactly is a Host Pool? Host Pools are a collection of one or more identical virtual machines in the AVD Environment, and it's a Host Pool that's the heart of AVD. Session hosts are a member of it, App groups are connected to it, and workspaces are associated with it. The reason why a Host Pool is a collection of identical virtual machine session Host is because a Host Pool should be created for a specific use case or user persona to consumer's resources. For example, you would have a Host Pool for the Accounting Department and a separate pool for those field sales folks. 

This is because the applications that each group needs will be unique and so to will be their behavior and performance requirements, which means that their configurations and properties will also be different. Each Host Pool will also be unique because of the number of users in that use case and they'll require different amounts of user density and different numbers of session hosts, which also means different configurations and properties. An example of this is in the join options. If you have domain join session host and Azure AD-joined session host, they will need to be in separate pools because the RDP property configurations are very different.