Routing and bridging are functionally very similar, but they have significant differences. Routing, the process of moving packets between two or more networks, is one of the primary functions of a firewall, and most of them do a good enough job at it to make routing seem transparent. With a minimum of configuration, pfSense is able to route traffic between your local network (LAN) and the internet (WAN). Little additional configuration is needed to add other local networks. Firewalls, however, initially only know how to route traffic to the networks directly attached to them. For example, if you have a router connected to one of pfSense's internal networks, pfSense will not know how to route traffic to any nodes attached to the router unless you define a static route for it.
Bridging is not something that is done in typical network configurations. Bridges...