Bark mulch is a long conversation but it should be replenished every year to two years with a new layer. Unfortunately, it can harden in beds from watering and aging and form a layer underneath similar to a cork board. Hence, the water from sprayers or above-ground sources will not reach the roots of plants and shrubs. A good compost (NOT the compost from the landfill which is rife with weed possibilities) is nutritious, holds in moisture, is good looking and, if it blows a little onto the lawn (unlike mulch) it is a good thing! Go for it, "Just."