What is the goal of this project?
To create, improve and maintain a high quality set of developer tutorials.
Do I have join this project to contribute to the Tutorial pages?
Not at all, but it helps to centralized the effort and keep duplicate work to a minimum on the Docs Wiki. See below, How much time...
Why do I have to sign up on for the project?
You are letting others know how you will contribute, even if it is in a small way. It will also add the discussion page to your Watchlist if you check the 'Watch this page' before you post. This is so you will receive news and more user notices of involvement in the project.
How much time is involved? I'm just too busy right now.
Your time and contributions are always appreciated! One small task might take only 5 mins a week of time (e.g. adding one page in a Tutorial series to your Watchlist to track changes). See above, Do I have to join to contribute...