I think its done to give Technet and MSDN paying customers some value add. This way they are getting the SP almost a month before people who didnt pay. I guess theoretically that lets them be prepared to deploy on the day of
also with a smaller number of more "tech" savy users getting the update first they can tell msft a lot about any problems out in "the real world" before millions of less savy users grab it.
that also gives them a chance to make any last minute fixes like a pre-sp patch to help with late issues.
and the time to get all the docs ready... and train an army of support techs who may have to take calls.