According to Google's Webmaster Central Blog, the switch will let Googlebot better understand web pages that use newer web technologies like HTML5. It's also making the switch because the tool that Googlebot uses to render webpages has been updated over time to behave more like an Android phone's native browser (Google Chrome) than an iPhone's native browser (Safari).
This could be seen as a symbolic move on Google's part, as iPhones have lost marketshare to Android phones over the past year. But in practical terms, the switch doesn't mean much. Google says that it should go unnoticed by most everyone, promising 99 percent of all websites won't be affected. Those could be sites already trying to block Googlebot from finding them. Anyone worried about the change can use Google's Fetch and Render tool to figure out if the mobile version of their site will continue to be catalogued as normal.