In an interview with CNET, Wales explains that the purpose of the site is not so much to bring day to day news about green topics, but rather a central knowledge hub for all things green.
"What we're doing is actually complementary to blogs in the sense that what blogs do is update you on a day-to-day basis. Also, blogs are engaged in political or other types of analysis, whereas a wiki becomes a touchpoint for the community, a place where people meet up and work on whatever the consensus is about a certain topic," he says.
It's also not just a green Wikipedia. The site itself describes three ways in which it is different from the regular wikipedia: the content is written from a green point of view; it is focused on what you can actually do, and it is more accessible and relevant to an average reader (see some comparisons here).
The question "do we really need another green site?" is asked in the interview, but it doesn't really apply here. I'm sure that Wales will be able to leverage his influence and the influence of Wikipedia's community to turn Wikia Green into an important resource for learning about the environment.