There are four extremely important reasons to mulch. The first is for water retention. Mulch stops the top of the soil drying out, keeps the soil moist, and can reduce the watering requirements. Mulching also prevents weeds and weed seed germination, which compete with plants for moisture and nutrients. Mulching also keeps the soil temperature constant, and using an organic mulch means you’re adding extra organic matter to the soil.
Although similar, compost provides a different purpose. Compost is decomposed organic material and provides many essential nutrients for healthy plant growth. Compost is spread below a layer of bark or mulch or worked through a garden bed. This process adds organic matter into the soil itself which increases the soil’s capability of accepting, holding, and releasing water, nutrients, and air for plants.