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.
The unregistered version of Harry will let you play to level five only, so you should register if you want to play the full 15 levels of the game.
In the Extras folder you will also find the Harry Level Editor. While this editor is a beta, meaning it's still being developed and tested, if you register your copy of Harry, you will be able to use the editor to create your own worlds and adventures for Harry.
Beyond that, you will receive a License Code that removes the shareware notices and personalizes Harry with your name, giving you a legal copy of Harry on your Macintosh.