Quite simply, because plants are nature's air freshener -- they continually improve a room's air quality by increasing the oxygen and removing pollutants. Easy-to-grow spider plants, philodendrons, and golden pothos are the most effective, according to a NASA study.
(cited from http://www.realage.com/. A good site to check out.)
(cited from http://www.realage.com/. A good site to check out.)