Yep, as mentioned above it's to give the APU a few minutes to warm up after a long leg in cold air to help it start better. The trick is to make sure to do it below 10,000', or more specifically below ~250kts, so the APU doesn't spin fast enough to begin the auto-start sequence but without the starter engaged.
Some 747 Classics have the APU switch connected to the ground safety relay and so it won't let you open the door before touchdown though.