How are landfills a good thing?

Answer 1

they are not good.

Answer 2:

Well, another way to look at it is that landfills are a method of carbon sequestration. All the plastic bags and other un-recycled materals that don't decay lock up carbon for millennia to come and keep it out of the atmosphere where it causes global warming. The organic materials that do decay, can be used to generate methane for energy use. Finally, at some point, landfills may come to be seen as mines of valuable materials that can be tapped into when natural stores of raw materials become rarer and more difficult to extract.