Buying plastic

I don't understand people who buys plastic bags for the garbage bin. It's so not necessary. I mean we have like tons of plasticbags lying around in the kitchen, and Marion tells me to buy plastic bags for the garbage. Such a waste.
One of her reasons was that otherwise we would have to change the bags more often, and everybody would be to lazy taking them out, so it would just lie around in the kitchen. Well, that would mean no change in this case. Everybody are alraedy too lazy taking them out...