Knowing where you are these days is about as difficult as reading a clock. So why would anyone bother with an expensive sextant that needs the support of at least two almanacs (or a celestial navigation calculator), clear skies, and a lot of practice?
Back-up of your electronic systems is one obvious answer, but there's more. Tremendous satisfaction comes from knowing and practicing celestial navigation. Taking sights is fun. At sea, it gives you a daily project. It keeps you in touch with the sky at night, and can make you a whiz at star and constellation identification. And every time you successfully reduce a round of sights, you've accomplished an intellectual riddle. It's not earth shattering, but it makes you feel good every time. And it makes you a better sailor.