Just because you can see your website on your phone does not mean it is mobile friendly. What mobile friendly actually means is that your website has a specified cascading style sheet (CSS) that serves a special mobile design of your website on mobile devices. This is most often noted by the size of the device.
The mobile specified style sheet is also crawled differently and the search engine crawler bots look for this specific file. This way, when they do find your site is indeed mobile friendly they will index the mobile version in their mobile search. Without having this file, your site might still be indexed in mobile search but it will show up below sites that are built correctly.
What’s the advantage of being found in Google mobile search? People will find you more often which will bring more traffic to your website and ultimately, more conversions. It also provides a better user experience because with a mobile version it is designed for mobile users and it is typically easy to navigate and read.
As of July, 2013 71% of people use their mobile device to access their social media sites. This means if you are running a campaign on social media you need a mobile friendly site to provide the best user experience for your visitors.
In short, creating a mobile friendly website will help to increase traffic, search rankings and the number of conversions by providing a better user experience for your visitors.