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.
No, I don’t use Windows. Steam is built using qt which my whole desktop is built with, the qt devs like snapping windows and I like them too :)