They've done it with other systems' controllers as well:
https://www.windowscentral.com/steam-now-officially-supports-nintendo-switch-pro-controller
Valve tried to radically redesign the gaming controller with its own Steam controller, but for a lot of players it never quite caught on. Fortunately, Steam is a very flexible OS and it's pretty easy to use your controller of choice. Like the PS4's Dualshock 4 controller, for example: Valve just...
www.engadget.com
Basically Valve wants Steam users to be able to fully configure and customize controllers of their choice, and use them however they wish, regardless of how well the OS or PC games support those controllers. Allows users to cut down on the amount of 3rd-party apps needed to get non-standard, non-Xinput controllers working on PC as well. That's the idea anyways. Some 3rd-party apps continue to have value, like DS4Windows and Joy-To-Key. I think it was a few years ago when Valve started this massive push for Steam to support an epic f*ckton of controllers. It might have been born from their efforts on Big Picture Mode, Steam Link, and their own Steam Controller. The configuration options are there if you need them, but most people using controllers on PC these days are using Xinput controllers, which has been the standard input API for a good long time now. Almost all games come with support for Xinput devices. So the games have all the built-in controller configuration one would usually need. Some non-Xinput controllers naturally work on PC, like DualShock 4, DualSense, Switch Pro Controller, and the Switch SNES controllers, but usually you need a 3rd-party app to be able to properly configure and use these controllers the way you want. And still other controllers like DualShock 3 don't work on PC at all without some kind of 3rd-party software, if I am not mistaken. Steam is trying to be a one-stop-shop for that kind of stuff, it seems to me.
Since I use my PC to play all kinds of games, and not just the ones where mouse and keyboard is the best control scheme, I use a large variety of controllers on my PC. Since I game on both consoles and PC (with PC being my main), I tend to have a lot of controllers lying around that I can easily use on PC, spanning across multiple console generations. I've never really ever had to use Steam's controller configurator. I use Joy-To-Key to map keyboard keys to controller buttons if I wanna do **** like Steam screenshots (which I have set to F12), for games that don't support controllers 100% (like Hydro Thunder), when I wanna make custom control schemes for certain games (like playing Cuphead and Sonic Mania using a Sega Saturn controller), or when I want to use an Xinput game on a non-Xinput controller. When I use controllers like SNES Classic Controller, Switch SNES controller, or NES controller on PC, I'm usually doing it to play emulators, so I'm not using Steam there either. So I can't really say how comprehensive or helpful Steam's controller support is. I think I do use Steam to treat my Switch Pro Controllers and Wii U Pro Controllers as Xinput controllers though.
In any case, this only helps you if you are playing games through Steam. Steam controller support doesn't affect games you're running standalone on the computer, or through another launcher, or with other software like emulators. For most people playing modern games on PC, which they are most likely doing through Steam, having controller configuration options built into Steam probably has them covered well enough. I'm sure the number of controller-using PC gamers using non-Xinput controllers is very low. Although I think DualShock 4 saw surprisingly widespread usage on PC. I know some people who actually prefer it over XB1 controller. Anyways yeah, it seems weird for the Steam client itself to have controller configuration options, but there are situations where it is probably quite useful.
