Because my phone is also an alarm clock. I have it on the bed stand, which is too far from a plug for a charging stand
One more reason for you charge your iPhone during the night. What happens if the battery drains and the iPhone turns off during the night? How will you wake up in the morning?
Yes, that is not something that should happen a lot (except in 3.0, since it's a beta) but it is something that could happen even in 2.2.1, an app can get stuck and keep running draining your battery.