Simply the fact that more and more people now own smartphones and tablets and connect to the Internet using these devices. As a merchant you must need to serve those customers to achieve a better growth rate.
Don't your realize that mobile commerce is developing at a significant rate. Given the rapidly growing number of smartphone and tablet users, this is reasonable, yet there are still a considerable measure of e-commerce retailers that haven't been staying aware of the times are as yet serving up unoptimized websites to mobile visitors that can undoubtedly push potentials customers away.
Having a mobile website is non-negotiable requirement, there's no question about it. But having said that, there are always opportunities to differentiate one's brand with the competition and perhaps engage and retain current customers using native applications. The native mobile app will provide a mobile centric experience to your customers.