Source: Why make a website mobile-friendly? — Webmaster’s Mobile Guide
There are outstanding statistics that give good reason to make a website mobile-friendly. In the USA, 94% of people with smartphones search for local information on their phones. And of those 77% of mobile searches occur at a place of home or work place, where desktop or laptop devices are likely present.
If that is not enough reasoning to have a mobile-friendly site, than the change Google made this April to its search algorithm should
convince you.
Google made a huge change to its search algorithm (effective on 4/21/2015) that affects all sites that are not mobile friendly.