It seems common knowledge that promo copies are used to dispatch new material over the internet... for example, U2's new album is already available everywhere, but it won't be released for 2 weeks!
So, if the new album is going to be released in February and promo copies go out on January, and you happen to see a mp3 copy of the album somewhere on the net....
Will you check it out or not? Be honest...! Sure, we'll all buy it, but will you wait for the real thing or not :D