Gardening is unnatural. Most popular plant species are not naturally native. Cultivars are products of unnatural selection and breeding. Most plants like unnatural watering. Some enjoy unnatural fertilizers. Removal of their detritus is unnatural. So is the replacement of their detritus with mulch. Actually, most of what happens in the garden is quite unnatural. 

Ironically, gardening is how many of us incorporate more nature into our lifestyles. Much of our effort compensates for what plants naturally crave but unnaturally lack. Watering is necessary for plants that get rain through summer within their natural ecosystems. Mulch might be nice for plants that naturally benefit from the decomposition of their own debris.

In nature, most plants benefit from their own debris. They enjoy the nutritiously decaying organic matter and enhanced moisture retention. Their shallow roots enjoy the insulating effect on the surface of the soil. Debris of some plants excludes other competitive plants. Mulch is not a perfect substitute for such detritus, but partly compensates for its removal.

Although it would likely be an asset to the plants that produce it, the detritus produced by most plants can not stay in the garden. Much of it is too abundant. Some is too shabby or too coarsely textured. Some might become combustible as it accumulates. Diseases can overwinter in some types of debris. Mulch is more sanitary, neater and less combustible.

Most mulches are organic matter of one sort or another. Compost does not last long as a mulch, but is appreciated by most plants. Uncomposted chips from a tree service occupy nitrogen as they decompose, but are more effective for inhibiting weed growth. Products that are available as mulch at garden centers are more refined, but also more expensive. 

Some dense ground cover plants perform something like mulches. Many consume more moisture than they retain though. Also, they can retain bits of potentially infectious debris that falls from diseased plants above. Nonetheless, they insulate the surface of their soil, and inhibit or exclude weeds. Gravel over ground cloth is inert, but requires no watering.