Mulch has many benefits
The major reason gardeners use mulch is to snuff out weed seeds by shading them. This allows the roots of desirable plants to access soil, water, and nutrients without undue competition. Mulches free of viable weed seeds—such as leaves, good compost, and wood chips—are best. Weed seedlings that sprout in any organic mulch are easily done in if you periodically fluff up and flip over the mulch with a pitchfork.
The second reason to mulch your garden is to conserve water. Organic mulches soften the impact of raindrops so that water can effectively permeate the soil, and all mulches, organic or otherwise, limit evaporation of soil moisture.
The benefits of mulch do not end with water and weeds. As organic mulches decompose, they promote healthy soil, which, in turn, helps fend off disease.
Mulches also regulate soil temperature, acting as insulation to prevent the alternating freezes and thaws that can heave plants out of the ground. Such ground-insulating mulches are especially useful in keeping the roots of newly planted trees and shrubs growing as long as possible into autumn, and keep the soil beneath evergreens unfrozen deeper and longer so that their roots can absorb moisture in winter.