Plants can be used for gardening and agriculture. Decayed organic matter (lawn clippings, leaves, manure) forms compost which makes a good fertilizer. See the picture below to see how compost is formed.
Mulch (often in the form of wood chips) is used in landscaping to enhance the look of an area but it also helps keep the soil moist, control weeds, and prevents soil temperature extremes.
Plants can also be used as a source of animal feed. Much of the corn and soybeans we grow are used to feed livestock.
Some plant compounds have been used in commercial insecticides. Other plants, such as cayenne pepper, have not been developed commercially but many gardeners use them as natural insect repellents.