Tesla seems to be adopting the "Wendy's strategy". Wendy's apparently sells excess hamburger as chili, thus somewhat compensating for daily swings in hamburger sales. Similarly Tesla is probably anticipating that their Gigafactory will also have unexpected swings in demand depending on vehicle sales and existing contracts with other battery suppliers.
By selling the excess Gigafactory battery production as battery based storage for homes, Tesla ensures two things: 1 - a better ramp up in Gigafactory utilization during the early years, and 2 - protection from unexpected swings in vehicle sales.
One small step for man, one giant stumble for mankind.