There's actually two settings for it.
1) Always on clock + battery + weather where only the pixels that are used for those pieces of information are powered on for limited power draw (about 1% battery/hr)
2) Night mode which basically turns off the entire screen except for the wraparound edge part, which has a somewhat dim clock/alarm so you can like put it on your nightstand or something and use it as an alarm clock without it being too bright.
I'll probably use the alarm clock feature, but always on screen during the day seems pointless when all I have to do is tap the lock button to see the clock anyway.
edit: actually I forgot a couple of other modes for the always on screen
3) You can set it to a few stock images, like a starry sky or a treeline, which shift around automatically to prevent burnin
4) A somewhat annoying screensaver thing that I don't know why anyone would use it