Obviously the exact details aren't published reading, but it's been widely rumored that WWW makes big promises of signing bonuses and affiliation with apparel companies like Nike to kids who agree to play ball. These bonuses may come before they enter UK, but they most likely are promises made for once they enter the draft. "Come to UK, play a year, get drafted by X at the latest according to my sources, and you have a Nike contract waiting for you. All you have to do is play for Cal and agree to sign with us when you are drafted".
It may seem like it's not worth the trouble, but once you see guys like Wall, and Cousins becoming superstars, and Davis looking like the next all-time great, you can see how it can be a lucrative relationship.