Updated: Jul 7, 2019
Bare ground is an invitation to a disaster, weeds sprouting almost instantly, soil drying faster than you can water it and forming a water-repelling crust, the soil itself losing humus and microorganisms. Mulch is a halfway step toward healthy soil covered with plants, but an essential one whenever you clear ground.
Remember, however, that all mulches eventually start growing weeds and will need to be replaced. Even bark on top of black plastic will eventually become a nursery bed for any plant that happens along.
Straw or other loose, airy substance is great for large areas or place you've sown seeds. You can move it around, it takes a long time to decay, and in thick enough layers it kills weeds.
Dry leaves start out loose but usually pack down, a good choice if you want to smother growth entirely. Don't use a thick layer around rhododendrons, azaleas and other shallow rooted shrubs whose fine roots are close to the surface. They'll die.
I like fine bark for sifting between established plants that haven't quite thickened up into a solid layer. It repels water for a while and needs a half inch layer or more to keep weeds from sprouting but it's a good choice.
My favorite, however, is moss. Those of you who don't live in the Pacific Northwest may be short on this, but if you have moss growing in your lawn, you can rake it out and put it in shady areas and it will take hold, expanding into a mat that can be lifted, pulled apart, moved and renewed with no negative effects on the plants around it. Gets a little brown in summer if the sun hits it but works well under perennials or anything that provides some shade.