Landfill gas is produced automatically if organical waste is disposed on a landfill site by anaerobic digestion like in a biogas plant.
Without active degassing system these landfill gases escape uncontrolled into the atmosphere. These emissions are boosting the greenhouse effect, because the main component of landfill gas is methane.
Because of these reason, but also because electricity and heat can be produced out of this "waste gas" it makes sense to suck off the gas from the landfill and valorisate the gas in modern cogeneration units.
In a lot of countrys it is obliged to install at least an emergency flare to be able to flare off additional gas quantities or to burn the gas in case of engine failures.