Organic mulches, such as wood chips, must be three to four inches thick to effectively keep weeds down. Organic mulch eventually breaks down improving soil, which means you'll need to replace it after several years. Mulches save water in the Garden because they reduce evaporation from the soil surface. Inorganic mulches, such as rock, can also hold in moisture. Mulches also improve water penetration and minimize soil crusting.
Some people prefer to use spun or woven landscape fabric under wood chips or gravel.
Unlike solid sheet plastic, which is no longer recommended, these materials promote air and water exchange within the soil. Mulches also control temperatures around plants. Organic mulches are cooler than rock mulches in the sun. Rocks absorb and store heat for long periods, reradiating heat to plants. Organic mulches are often used to alter temperatures in other ways.