Being under a desk won’t do much if a nuke lands close but if you’re far enough away, it can at least provide some protection from falling debris. Marginally better than sitting straight up, anyway, and if everyone in a survivable-but-still-hazardous zone does it, you can slightly reduce the immediate losses.

Obviously that’s not the main focus of your comment but the whole “duck and cover” thing isn’t as ridiculous as it may seem.