The other really valid reason is linen. Kinda unrelated to sewing itself and it's not about stopping the stuff from crinkling (that's right-out impossible), but to make sure that crinkles don't always appear in the same place so the fabric has a chance of wearing down evenly.

Found this out the hard way because my linen duvet covers are oversized -- nominal size is correct, but they're made for down blankets, not flat ones. Blanket slides inside, generally towards the bottom, leaving a fabric flap on the top that really tends to crinkle as you sleep, wash, hang up, the crinkles don't straighten out, exact same crinkles appear in the exact same spot and get chafed while sleeping, rinse and repeat for two years the first hole starts appearing, a month later there's more than you can be bothered to patch.

Luckily it was a simple matter of running a stitch down the length of the thing to shorten it a bit, but given that an iron and ironing mat (not a full table, mat is completely sufficient) is significantly cheaper than linen covers or just the material for them, definitely worth the investment and time.

Oh and yes linen covers are definitely worth it because moisture regulation. It's also nice and soft -- not in the silky smooth sense, it has definitive grip to it. So are linen kitchen towels because they actually dry stuff instead of spreading water around. Half-linen is already a massive upgrade over cotton in that area and it's much cheaper (the main reason why full linen is so expensive is because it's a bugger to weave, not because the yarn is that much more expensive. Weaving linen wefts into cotton warps OTOH is pretty uncomplicated).