What is a plant that is useful to humans in a way other than to provide us with oxygen? (For example, to provide us with food). What type of plant is it? Where is it found? What role does it play in the ecosystem, etc.?
Plants play an increasingly vital role in the life of human beings. In addition to the incomparably important function of providing oxygen, they are also essential sources of nutrition and medicine.
Plant extracts have been used for medicinal purposes for as long as homo sapiens have walked the earth. Ancient Egyptians chewed on bark, jungle tribesmen in Asia used plants to relieve pain and to depress their appetites in the absence of more palatable options for consumption, Somalia tribesman even today chew khat for its neurological effects, particularly appetite suppression and hallucinogenic "benefits."
The Alderleaf Wilderness College, located in the Cascade Mountains north of Seattle, Washington, provides a useful list of plants and their use by people for health and nutrition purposes, for example, the use of elderberry plants in medicine and vitamin supplements, and to ease anxiety. According to the college's website, blue and red elderberries are rich in vitamins A and C. Websites focused on the health of the Amazon jungle emphasize the importance of plants unique to that region for the treatment of malaria, prostrate enlargement, respiratory illnesses, anxiety, and much more. The Cinchona Tree, according to one such source, is used to cure malaria, and the Calvillia is used for treating infections caused by viruses, bacteria, fungi, and parasites.
The question asks about the location of these plants, and their role in the ecosystem. That question raises a very important issue. Many of these plants are native only to rain forests and jungles in Latin America and central Africa. Despite the value of these plants as sources of medicines -- including in the struggle to find cures for cancer -- the forests and jungles are under constant threat from governments and companies seeking to exploit the land for more profitable -- at least in the short-term -- purposes. The construction of highways through the Amazon, the use of "slash and burn" farming techniques by subsistence farmers in South America, the development of land for commercial purposes and for housing, all contribute to the gradual but steady decline in the size of the jungles that produce these plants. As with endangered animals, vitally-important plants are regularly threatened because of man's unwillingness to interact in a more constructive and sensitive manner with his environment.
One of the most beneficial plants is the cocoa tree, which provides substances used in the treatment of anxiety, kidney stones, infections, fatigue, and more. Cocoa plants, more than most, tend to be protected because of the very sizable market for the proceeds of that particular plant.