No additional functionally by forcing Cellular on the Watch. As mentioned above, LTE really burns through your Watch's battery. The Watch want's to use Bluetooth to your iPhone, then Wi-Fi, to your iPhone and finally LTE if it has to as a last resort. The Watch really tries hard not to use LTE.
LTE should only be needed when you are out and about without your iPhone and even then it wants your iPhone to be available.
For example: SMS/MMS text messaging requires that your synced smartphone must be powered on so SMS/MMS messages may be forwarded to your watch. iMessage must be enabled and your smartphone must be connected to the AT&T network via either a cellular or Wi-Fi connection. If using a Wi-Fi connection, your smartphone must have Wi-Fi calling activated.