To create, improve and maintain a high quality set of developer tutorials.
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...
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.
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...