Border router vs BBR

[dakhnod]

The way I understand it, a border allows routing packets from an ethernet network to a thread network by bridging the two.

Following that logic, I have a question about two different Thread partitions:

A thread device sends a packet from Thread partition A, addressing a device inside partition B.

The border router from A should forward the packet to the connecting Ethernet network, where it should get routed to Border router B, and then to the device inside B.

So, according to my logic, a BBR should not be needed, since both border routers (A and B) handle the briding automatically, right?

What benefits would a BBR bring in that case?

Thanks for your time!