I do this, though I hadn't heard of other people doing it before and use it slightly differently. Mine is set to 9:30pm and it is a marker point for me to use when keeping track of time late at night.

If you have the kind of schedule where you get home at 6pm and use a computer until late, then it's easy to lose track of time until it's suddenly 2am. Having a marker point closer to your sleep time (say 9:30pm for a 11pm bedtime) helps in keeping up your internal clock and signals when you need to make schedule modifications in order to finish things up before you need to sleep. I think this is also a good idea to create the form of alarm that will get you to go to sleep but also avoid Hyperbolic Discounting at the last minute.