Why is PPPoE to PPPoA Bridging useful?

Sometimes you want the modem functionality of the device, but not its routing or firewall capabilities. For example, maybe you want to use PFSense as your router/firewall but you only have the device you mentioned. By putting it into bridge mode, it allows you to use PFSense as your PPPoE device, giving you all the flexibility and functionality of the PFSense firewall withotu having to NAT to a NAT.