A post on Steam explains how Valve is helping developers and publishers to better understand their audience by providing new reporting on what sort of controllers are being used in their games. This comes in a new Steamworks Sales & Activations dashboard for Steamworks partners. Here's a bit on the significance of controller support:

Let’s start with some figures for a sense of scale: 48 Million players have used a controller in a game on Steam, with about 10% of daily game sessions being played with a controller. Of course, that controller usage isn’t distributed evenly across all kinds of games. Many RTS games, are hard to play with a controller of any kind and tend to have a much smaller percentage of players using a controller (frequently below 1%). In contrast, many sports and fighting games see over 70% of sessions played with a controller, while racing and skating games can easily have over 90% of players using a controller. On the action and adventure fronts, most 3rd person adventure games see 40-50% of players using a controller, and the majority of FPS games are solidly in the range of 7-8%.