There are several reasons to want to reduce the number and size of landfills. First, landfills are very oxygen-poor, which promotes the growth of anaerobic bacteria. The bacteria produce methane, rather than CO2 when breaking down waste. Methane is a much more potent greenhouse gas than CO2 (though it doesn't stay in the atmosphere as long). Second, greenhouse gases are released in the extraction of petrochemicals. Reducing the new plastic created would reduce this effect. Lastly, landfills can have an immediate negative effect on the environment surrounding them if solvents and heavy metals leach out of the landfill.