To avoid the crowds.
Ohhhh. Thanks for the explanation. I was raised with the expectation that you go have fun once you've met your commitments, not before. But gee, I guess that would mean longer lines! :)
(Not that I've ever been to Disneyland/World; it never appealed to me as a kid, and I asked my husband once about going as an adult, since he had been; he said that when the park opens in the morning it's like the running of the bulls at Pamplona, and that was all I needed to hear. None for me, thanks!)