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.
I know this shortcut, and it resizes the window to half-screen. Got any shortcut that does the same, without resizing?
Check out Multitasking settings and see if any of those do what you are asking.