Mother Nature applies mulch every time leaves, twigs, fruits, or other organic matter drop from trees, shrubs, and other plants. These materials decompose, enriching the soil below. Landscaping mulch does much the same thing, only in a more organized, deliberate fashion.
Visual continuity. Unify the look of your yard by using the same type and color of mulch throughout your landscape.
Soil insulation. Keep the soil in your planting beds warmer in the winter and cooler in the summer.
Weed control. Keep weed seeds, which need sun and room to grow, crowded and in the dark.
Moisture retention. Slow evaporation of water from the ground.
Soil-splash reduction and erosion control. Keep soil where in place so it doesn't wash away or splash upward and drop soil-borne diseases on plants.
Tree-trunk protection. Protect trunks from damage caused by lawn equipment getting too near with mulch placed around the trunks of trees to the drip line.
Soil-compaction prevention and soil crusting. Prevent soil compaction and/or crusting so water can filter through easily.