It's so that if the device buttons are pressed accidentally (such as while the device is in a bag) the device doesn't turn on and start paging around.
What is the purpose of the "lock" that appears at the bottom of the first screen when powering-up the Glowlight? On the HD+ I have the option of programming in a security code, which must be entered before anyone can pass the first screen.
As far as I can tell, this security feature doesn't exist on the Glowlight. Since anyone can unlock this screen simply by swiping the lock, what's the point of having to "unlock" this screen before you can go on?
It's like having two on/off switches...
Having to swipe after pressing the button helps ensure that the activation wasn't accidental.
After all, I'm sure you would be unhappy to find that an accidental bump of the power button enabled furthur josteling of the nook in your bag or briefcase to change page in your book, close you book, open another book, archive, or even delete your book.
For this same reason your smart phone requires some kind of swipe or touch after waking up before you can actually use it. We don't want to be butt-dialing any 900 numbers after all.