Ambrosia offers software on a "try before you buy basis". Supporting us by registering our products -- assuming you like them -- is the only way you can ensure that we will continue to produce more high quality software.
There are more pertinent reasons why, as the unregistered version only allows you to play to the first Boss of the game. Once you beat him, you are unable to proceed further without registering. Also purchasing a registration code will allow you to use the game editor.
Beyond that, you will receive a License Code that removes the shareware notices and personalizes SketchFighter with your name, giving you a legal copy of SketchFighter on your Macintosh.