Because devices will accidentally wake up when in a pocket / bag / etc, and you'd prefer not to feed random touch events to whatever program you're running. Of

Because devices will accidentally wake up when in a pocket / bag / etc, and you'd prefer not to feed random touch events to whatever program you're running. Of course sometimes this won't be relevant, and you could do away with the lock screen. E.g. the Eee Transformer requires the usual Honeycomb unlock gesture when it wakes up undocked or in the dock by pressing the power button, but not when it's in laptop mode and is woken up by the lid being opened.