Thanks, that's very helpful. If I understood correctly, it sounds like a hub consisting of 2 brokers would act very similar to just bridging 2 brokers with a parentId.
Unfortunately, you can't bridge two brokers and have them bridge to the same parent identifier. A broker can only have one outgoing bridge (it can have an unlimited number of incoming, aka brokers bridging to it). That is the reason the hub exists, the brokers are able to dynamically change who they bridge to based on the presence (or absence) of the other broker in the hub.
For a simple deployment of just 2 or 3 brokers, would there be any reason to introduce a hub? Is that more of an advanced configuration, once you start to scale out to many brokers across different sites and you want the resilience between hubs?
For two brokers, no.
For three brokers, yes.
By having a hub comprised of two brokers and a single spoke, if a single broker goes down, you are still guaranteed to have the remaining two brokers connected. If you had three brokers connected to each other without a hub, if the middle broker went down, the remaining two would be unable to communicate with each other.
Hope this helps,