Ornamental plants used indoors help purify the air and create a pleasant atmosphere. Many people position plants around the home in accordance with the ancient Chinese practice of Feng Shui. Outdoors, ornamental plants help beautify a landscape and create an appealing environment.

...
You can use ornamental plants indoors or outdoors.

Purify Indoor Air

...
The peace lily may help purify indoor air.

Synthetic materials give off volatile organic compounds, or VOCs. Home environments contain many synthetic materials, and are often sealed off from outside air for energy efficiency, making them susceptible to formation of these compounds.

The process of purifying air with plants is called bio-remediation. In controlled studies by NASA scientists, some common ornamental plants--the Boston fern, the bamboo palm, the ficus tree and the peace lily--ranked high in their ability to purify the air of the VOC formaldehyde. While the studies are not complete as of September 2010, the preliminary evidence suggests that people can use ornamental plants inside homes and apartments to help clean the air of VOCs.

Note that soil in potted plants may release mold and bacteria into the air unless you cover the soil with about 2 inches of gravel.

Feng Shui

...
Round leafed plants have a calming energy

Feng Shui is the practice of positioning objects, including buildings, furnishings and plants, to create a balance of yin and yang energy and encourage the flow of good energy, or "chi."

Ornamental plants play an important role in Feng Shui. Plants with round, floppy leaves, like African violets, produce yin energy and a sense of calm. Plants with pointed leaves produce yang energy. Place them in dark corners or other places where negative energy is high and positive energy may not be able to flow. Thick, leafy plants slow the flow of energy. Use them in long hallways. Avoid placing plants in the bedroom.

Beautify a Landscape

...
Hanging plants soften the facade of a building.

Gardens may be composed entirely of ornamental plants or combined with herbs, fruiting plants and vegetables. You can use a showy ornamental as a centerpiece for a landscape. Dogwood trees or crab-apple trees produce impressive spring flower displays. A sugar maple tree becomes the center of a landscape with a brilliant fall foliage display. Ornamental plants in hanging baskets brighten a porch and soften the facade of a building.