The Bridge (BR100) can do several useful tasks. Firstly, it is a transceiver and is the favoured piece which, when connected to your router and acts as the gateway for the Sonos system to tap into your network and access sources in the home network and out on the web. This frees up the other Sonos pieces to be located wherever you need them in the home and used wirelessly. Secondly, the Bridge can assist with creating better house-wide propagation of the wireless signals by acting as a relay or repeater. If you are having any reception issues in a part of the house, adding a Zonebridge can help. In fact, the more pieces of Sonos you have, the better your wireless network coverage will be, since they are all backing up the signal to one another.
A Bridge can also act as a wireless entry point for any computer which is struggling to receive good wifi signals. Just connect it to power near the computer and connect an ethernet cable from the computer to the Bridge. Set up the networking as if the computer was directly wired rather than wifi. This can also be done with other Sonos components - they all act as relays for the wireless network.