A wider screen on a laptop has the advantage of allowing a wider keyboard-- so smaller laptops (12"-13" and below) can have full size (or closer to full-size) keyboards, and larger laptops (15" or so) can fit things like a full numeric keypad (15-inch 16:10 doesn't even quite leave room for one, but 15-inch 16:9 does without inflating the screen bezel too much).

It's also probably not coincidental that a wider screen ratio allows the manufacturer to sell less screen (in terms of area or total pixels) at the same diagonal size.