promo copies go to radio stations, magazines and other reviewers and people in a position to play the songs before release, in order to generate buzz so that people will be aware of it and buy it on the first day. this is a major factor in record companies' assessment of how well a band sells - how many sell the first day, week, month.
not all of these people keep the secret, however, and the ego boost they get out of being in-the-know by slipping the music out to their friends is the cause of many leaks. friends have friends, who have friends, who use p2p, and then, everyone knows. sometimes leaks are even intentional, though never too far in advance as the intention is to generate talk and interest, rather than freely give away the album to everyone.