Roaming: Forwarding vs. Serving Network Server?

I’m reading the LoRaWAN Backend Interfaces specification. Here’s a diagram extracted from the document that shows the roaming architecture:
Screenshot from 2022-05-06 09-22-39

It’s not clear to me why they make a distinction between the serving and forwarding network server. What does each one do? I would expect just one third party network server relaying messages to the end-device’s home server. Why do we also have a “forwarding” server, in addition to the serving network server that is relaying messages?