Mulch helps conserve and extend available water, protects the soil from erosion, reduces competition by suppressing weeds, moderates temperature extremes, and acts as a barrier or visible marker of gardening beds.
Be careful not to use too much mulch as that can create a layer that doesn’t decompose or allow for root growth. The soil can become so compacted that water and nutrients can’t penetrate to the roots. This is easily avoided by stripping off as much of the old mulch and then adding a fresh layer each year. Keep your mulch around 3 inches deep, making sure to keep it away from the trunk of trees or shrubs.