Rent the Runway took the web by storm when it launched in 2009. The site gave customers the chance to rent dresses and accessories made by famous designers. As the amount of website traffic grew, the company's founders decided to invest in mobile. Data showed that mobile visitors accounted for about a quarter of their traffic. The result was a new mobile site and an app. We take look at why Rent the Runway focused on mobile, how they divided up their resources, and whether you should be doing the same.