I was in Webflow for 3 months when one told me about it (:
It’s a rare shortcut in Webflow when you think about it. All the buttons and settings in this panel refer to real life HTML/CSS properties, one at a time. This button, however, is a shortcut to make right and left margins auto and set the display property to “block” (that makes your centering, by the way, if you wondered).
And I guess they can build it because it works every time. I’d like the same shortcut for vertical centering, adding a top 50% + translate v -50%, but that’s a bit trickier, could break things on certain circumstances. (for one, the parent must have a declared height somehow)