I’ve been using Linux for approximately 20-25 years.

If we’re talking about what one thing steam does well? Gaming!

It handles dependencies, and it makes windows games transparent to Linux users (you don’t even know it’s being emulated). In fact, from my understanding, some games run better than in windows in some cases

If you’re using controllers, steam input also makes games a lot more playable.

If we’re talking about the Unix philosophy though, what one thing does the Linux kernel do?

The Unix philosophy better targets daemons then user facing apps. In fact, if you look at xorg/xfree, one could argue it is one thing. But if you break it down into a different perspective, it could mean many things