Making people think you're smarter than you are. That's about it.

What do you mean, what knowing the day of week a date is useful for ?

Yes, that's what people typically use a calendar for. So you agree that this trick isn't useful for anything?

I'm reminded of the scene in Back To The Future II in which people in the 19th century scoffed that someday someone might "run for fun" despite the ubiquity of motorized transportation.