Well, really, our days fluctuate, so to be really scientifically accurate, we should shorten our days as the Earth's rotation speeds up and lengthen our days as it slows down. But for simplicity's sake, it's easier to have a leap day than to tell people "Today is 23 hrs. 54 min., but tomorrow is 23 hrs. 55 min." And hey, it could be worse: some calendars have a leap month!