There's no getting away from the smartphone. One study showed that right now in 2012, 47% of US adults carry a smartphone. What do people do on their smartphones? We check Facebook, or Google+, we chat, we play games, we read our email, we send text messages, and more.
Why is this important? You need your web site to represent you well when viewed on smartphones and other mobile devices. You need to give a great experience to your visitors, not one consisting of pinching, zooming, and clicking the wrong links.