[Source 2] The Gigafactory is simply about reducing cost per kilowatt hour really drastically. Once the Gigafactory reaches its full production capacity, Tesla’s goal is to reduce the cost per kilowatt hour by more than 30%. As Elon said, “It’s taking economy of scale as far as we can possibly imagine, to a very extreme level.” [Source 1]
Taking this risky step, building the Gigafactory, is a key factor for success in bringing a mass produced electric car on the market, success of Elon’s “Master Plan” and basically also for success of Tesla Motors itself. They expect to begin cell production in 2017 and according to Tesla, "by 2020, the Gigafactory will reach full capacity and produce more lithium ion batteries annually than were produced worldwide in 2013" thanks to the planned annual battery production capacity of 35 gigawatt-hours. [Source