Most homes waste huge amounts of energy, through air leaks, poor insulation, inefficient HVAC systems, and more. This is true of most new homes and virtually all old homes.
The result is utility bills which are higher than they should be, uncomfortable homes, and countless amounts of carbon emmisions, leading to new coal plants being built and more mountain tops razed to fuel those plants.
Fortunately, most of these problems are simple and cheap to fix. Spending a little money on weatherizing your home is guaranteed to save energy and money in the long run, and it's one of the best things you can do for the environment.