Wikipedia is basically an encyclopedia of everything, collaborated on by thousands of people, some of which cannot disclose the information they have because of fear from political, social, and/or judicial punishment. By keeping their identity safe, these people are able to share any and all the information they have without having to fear for their lives in any form; as far as the integrity of Wikipedia's information is concerned, all changes are reviewed painstakingly so being anonymous doesn't make any person's opinion anymore valid or invalid than if they had a publicly visible identity.
At this point, Wikipedia is so prevalent that it is no longer an encyclopedia - which it was never especially good at - and is now being used by lobbyists, PR agencies, etc. to advance and promote careers and policies. If there was no anonymity, it would help curb these practices.