Try it. Move your Steam window around. See how it snaps to the edges of your display(s)?
Why is Steam the only program (I know) that does that natively? I would love for ALL my Windows programs to do that (without installing 3rd party softwares/scripting).
EDIT : I’m on Win10.
Windows key + left or right arrow key will blow your mind.
That doesn’t really help in my experience because this state is not saved. You have to do tricks in order to get it saved like move the window a bit or resize it etc.
Check out PowerToys
Also up and down after left right