The reason is simple. This device, even though it has two Ethernet ports on the back, you can only use one port at a time, since it only receives one IP Address and this is used by the port that you currently have connected to the router.
 In order to use both ports at the same time, your ISP would have to send you an additional IP for the second port.

Also could be useful to use this feature in case you will like it to set up an link aggregation on the future

Also remember that the IP aggregation is by a extra monthly fee with your ISP,

https://arris.secure.force.com/consumers/articles/General_FAQs/SB8200-Link-Aggregation-Setup

Another reason is that every time that you switch the modem from one device to other you need to reboot the modem in order to refresh the connection and et the modem recognize the device that is connected to it.