If your modem has a built in router there is no need for this system. In my experience, routers built in to the provider's modem aren't as feature rich, and offer slower performance. Not to mention that you have no control over the router settings without calling the provider to fix them, which is usually a charge. Providers here also charge a monthly fee for you to enable the router portion of their modem, which you can avoid by simply buying your own router.
Do NOT put multiple routers in your network unless you have a special reason. I've had to troubleshoot many customer networks where this has been the case, and it always causes issues.