And not just that, it's also future-proof, so if calibre ever "dies", and you find your old backups 10, 20 years after, you'll still be able to get all the books out!
This is not true for many programs, even opensource, where data gets stored in many different obscure ways that cannot be retrieved/restored later, sometimes even if the software still exists (but is a few versions newer and doesn't recognize old formats anymore).
