Those make little if any difference, basic thermal dynamics - they're cooling the exterior of your computer, when it's generating heat on the interior. It's like putting a fan outside your house expecting the internal temperature to go down... All they really do is keep the outside shell cool, if that's something you desire then go for it.
So no, they are not needed and are kind of a scam. Especially, stay well away from any that plug straight into your laptop, a Logitech should be okay but a lot are cheap nasty electronics that have a capacity to damage the laptop.