If you get it deep enough it prevents all but the most desperate of weeds. But it's main attraction is that it helps the soil retain it's moisture.

Mulches are a labor saving device for the gardener. A layer of mulch will help prevent the germination of many weed seeds, reducing the need for cultivation or the use of herbicides. Mulches also help moderate the soil temperature and retain moisture during dry weather, reducing the need for watering. Mulches protect the soil from the impact of raindrops that can cause crusting. Crusting can prevent the germination of seedlings.

Mulches also can be used to enhance the look of your garden. Many bark mulches provide uniformly rich brown color that contrasts with the plants. The mulch helps keep plants clean by reducing the splash of soil onto leaves during rainstorms, and helps infiltration of the rainfall into the garden.

from Benefits of Mulch, Natural Resources Conservation Service

Another good mulch material is waste wool from shepherds. It retains the moisture VERY well, and as it decomposes it adds lots of nitrogen and sheep dung to the soil--it's a mulch AND a soil amendment.
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"If you want yor children to be bright, read them fairytales-- if you want them to be brilliant, read them even more fairytales" Albert Einstein