The MysteriousStranger is the default name given to yet unidentified visitors. It is meant as a gentle incentive for contributors to our wiki to identify themselves, preferably with their real name smashed together (ie. JohnDoe?, not John Doe).
Why leave your name? A wiki is a collaborative effort from the part of a community. We want to know whom to be grateful to for every contribution. Leave your name and you'll feel the good karma flowing towards you.
Why smash your name together? Because if you do that it will become a wiki link leading to a page where you can also enter a few things about yourself. See for example how smashing MysteriousStranger together created a link which led you to this page.