What Constitutes a Unique Article?

There seems to be a lot of stress on 'uniqueness' of articles. What is the reason that 'unique' articles are valued? Does google somehow realize that this is non-unique text (and rate it lower somehow), or is it simply that if someone searches for some keywords that are in the article, that now many sites (which use the same article) will appear?

Following up on that,
I'm wondering if anyone knows what constitutes a 'unique' article in google's eyes. I.e. if you wanted to change an article to make it unique, how many words would you have to change? Does structure matter?

From what I have read, a good rule of thumb is to change at least 30%... and of course rename the title.

i would go for 50% Uniquness at least...