Why is it there? Because all SE sites have that button, even Bicycles. I think it's just one of those defaults that's just there, especially since Stack Overflow is still the "baseline" for most SE sites' feature set. That aside I guess it could be used for fixed-formatting. And I have seen some legitimate uses showing CSS/HTML used in web design/accessibility questions that aren't really about implementation, the code's just there to help you visualize/understand what specifically they're talking about.
I'm not really sure we need it, but I'm not sure if it's confusing people either; it does read "preformatted code" rather than "code sample" in the tool tip, and I think it's inevitable some people will assume if web design is on topic CSS/HTML/whatever are on topic; I haven't seen enough of such posts (yet) that we can't just close them, anyway..