I hope it becomes clearer as the idea grows and the promotion for this new cause to use Twitter gains steam.
@1milliontweets is an account for a charity that is asking for $1 to tweet through their account. They hope to reach 1 million tweets and their by write a book and raise some money for their causes. The Twestival for water charity was really well supported and I feel went with the theme of twitter for social gathering. This seems to cache in on the million dollar homepage that got a lots of publicity.
For a charity fair enough, but for twitter in this way I am not sure how good it is on a new account. When I wrote this had 173 followers, so the value to people sending messages at the moment is around that mark, where as after a few hundred thousand messages $1 will go a lot further for a company. Also with twitter being as it is you could just @1milliontweets or #1milliontweets in your message and it will be almost as effective.
Anyway we can watch and wait for how this works for them.