1.) They are easy to mail to relevant people.
2.) Cutting them from the inserts provides excellent exercise for fine motor skills.
3.) They save shoppers money and/or provide them with free goods.
4.) Coupon caddies are GORGEOUS. I am pretty sure that is not the correct term, but it just came to be and I LOVE IT. (I also love my coupon caddy; it is from the dollar store and completely emblazoned with butterflies.)
5.) Via a buy x, get y free or a coupon that makes an item free, I am sometimes encouraged to try new and different things.