Composting is a vital part of the diversion of waste from landfill. Given that an estimated 45% - 50% of of landfill waste is organic, there is a huge loss of potentially valuable material completely underutilised. It is also the organic material that breaks down and generates most of the methane gas that comes from landfills. Methane gas is more destructive on the atmosphere than carbon dioxide.
Here is an excellent description of composting