A layer of mulch helps conserve soil moisture, lower soil temperature around the roots of plants, and helps nourish the plants.
Drip irrigation works well with mulch. The composting that occurs at the moist soil surface with organic mulches also nourish plants, earth worms, bacteria, and other valuable soil life.
Mulch also reduces soil erosion and the growth of weeds, which compete with plants for water.
Mulch should never be applied all the way to the trunk of trees or shrubs as this may result in crown rot disease or other problems.