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Strictly speaking, there is no need for several networks. What you read is just a suggestion. You can put all traffic on a single network, or set up a much more complex network configuration. A moderately complex example:

Having said that, the management network carries traffic between OpenStack components, in particular message queue, database and inter-service API traffic. Since compute nodes have some OpenStack components, they need to be connected to the management network.

The term “tenant network” normally denotes a virtual network managed by Neutron to which instances, Neutron routers, Neutron DHCP servers etc. are connected. What you probably mean is often called the VM network. It implements the tenant networks and carries traffic between instances (so-called east-west traffic) and between instances and the outside world (north-south). Depending on your deployment, some of the former and all or some of the latter traffic flows through the host that contains the central Neutron components like the L3 agent, which implements routing.