It's called creating a false sense of urgency. They hold back the good seats to make people think that tickets are selling really well and they better buy them soon because before you know it they'll be gone. Most shows do this.
People usually have the impression that they should buy tickets far in advance so that they can ensure the best seats, when the fact of the matter is that producers sell the worst seats first, and release the better seats as the date of the performance approaches, with the unsold premium seats being released a few hours prior to the performance.
"Winning a Tony this year is like winning Best Attendance in third grade: no one will care but the winner and their mom."
-Kad
"I have also met him in person, and I find him to be quite funny actually. Arrogant and often misinformed, but still funny."
-bjh2114 (on Michael Riedel)