The "Go" button that is now visible on en.wikipedia.org is my nefarious creation. It allows you to visit a page directly by typing its name, e.g. "Main Page"->Go will put you directly on Main Page.
The currently active version is not the latest one. I have recently changed the behavior of the button to try some capitalization variants, so "main page" and "MAIN PAGE" work just as well, and to run a normal search if no direct hit can be found. That improved version should be activated shortly.
What's the use? Well, if you have certain pages you visit frequently, or jump back and forth a lot, this makes navigation easier. Anyone who has in the past run a search just to visit a particular page should appreciate this change.
Regards,
Erik