Because you need a license key. I guess the developers use this as a way to avoid spreading free versions everywhere, so every user register here and they can send information to them if needed when the final release is published.
To get not the watermarks, you only need to register in this site, and then click to ask for a product key (on top on your personal info. Then, open TT, and in Help tab, and there enter your activation key. Then, your software will be registered, without watermark :)