Paul,
You'll see that each remote site must have a different subnet mask. This is used to place phone in the proper site via their IP address. The teleworker site is assumed to be the catch-all site for IPs not contained in any other site's subnet mask. Thus, your DHCP server needs to hand out IP addresses appropriately to place phones in the desired site.
I'm also reading into your question an implied question similar to, "Why would I want to create a remote site?"
Typically sites are geographical (different city/campus) with different area codes as described by Sanjay. But, they could be logical (different building/floor/section of building?).
Logically, (in 8.6.4?) you can control which phones in a site can uses PSTN connectoins in other sites. Thus, you can have one floor use one gateway, and another floor use another gateway.
As noted above, sites affect routing. By grouping phones by site, the BE3000 can control the ability to do tail end hop-off (calling from a phone in one site out a gateway in another site). In some countries this disallow this by law. In this case, it is typically because the remote site is in a different city.
Also, emergency dialing is controlled by site. If one has an FXO port on a SPA8800 at a site designated for emergency calling, then emergency calls from phones in that site go to that FXO port. If you have a 2901 in the remote site, then emergency calls from phones in that site go to that 2901.
