Nowadays email is being read more often on cell phones than laptops or desktops. This means it's more important than ever to optimize your emails to look good and be easy to read on mobile devices.
According to a recent US Consumer Device Preference Report, in Q4 2013, 65% of marketing emails were opened on a tablet or smartphone first. That's some pretty impressive numbers. Two out of three emails are first read on mobile devices. That means it's crucial to design emails specifically for mobile devices.
Another impressive statistic, is that email opens for mobile devices is still increasing year-over-year vs. desktops. From Jan 2013 to Dec 2013, email opens for mobile devices jumped from 43% to 51%, as reported by this Litmus blog post.