Valve are trying to give developer a bit more data on what controller / gamepad they're using, which is a good reminder for developers to make sure they are testing for more than just a mouse and keyboard.
In the announcement, Valve mentioned how "48 Million players have used a controller in a game on Steam, with about 10% of daily game sessions being played with a controller" - that's a lot of people, and of course it does heavily depend on the genre.
Showing off some sample data from the new reports system, it gives developers a pretty nice clear picture of what hardware people having in their hands (or could have, if developers added better support). Developers will see how many people have played their game with a gamepad, it gives a little league table of popularity to show what exact controller players have been using, daily active users that hook up a controller, graphs over time and much more.
The data that Valve showed for games that weren't named, clearly showed that the Xbox Controller was always top of the list. Not surprising though and I doubt anyone will be shocked by that.
The big example is below to save you a click if you just want to see quickly (click it to enlarge):
Seems like the data has surprised even the folks at Valve, who mentioned that "a lot of players like to play a large cross section of games on Steam with a controller – which is something many people, including those of us at Valve, find a bit surprising".
Nowadays on Steam, if a game can support Steam Input it ends up getting access to a lot of hardware support. As Valve adds support for more, developers pretty much get it free with Steam Input. There's also Gamepad Emulation which is the more basic form of it. For more details see the info here.