Pundits have said it for a long time--Nature abhors bare ground.  Left alone, untended earth will quickly become covered with small plants, then bigger plants and, eventually trees.  This succession depends on geography and topography, of course, but unless something is seriously wrong with the soil, succession will take place.

In the garden you want to encourage the desirable plants and discourage the weeds.  The best way to do this is to prevent Nature from covering the earth with random seedlings.  Mulch is a big help in keeping things under control.  You can mulch with just about anything--pine straw, compost, semi-decomposed leaves, bark chips or shreds, cocoa bean hulls, gravel or even crushed shells.  Pine straw, compost and semi-decomposed leaves have the advantage of being free, renewable and generally available.  I especially love pine straw because it is soft, stays in place and smells good.

Mulch thickly to a depth of three or even four inches.  Be careful to avoid piling mulch up around the bases of trees and woody shrubs, as it hastens rot and disease. 

In dry areas, a good coating of mulch also helps preserve moisture in the soil.  Poor soils are made richer and good soil is maintained. 

Once I discovered the joys of mulch as a young gardener, I reduced the heavy digging in my garden.  Now I know that even on those days when I prefer to sit on the back porch with a trashy novel, the mulch is working hard to loosen the heavy earth in my garden beds.