From the plant's point of view, oxygen is a waste product in the process of photosynthesis, whose purpose is to provide food for the plant in the form of sugars (especially glucose).
Photosynthesis essentially uses light energy to convert carbon dioxide and water into carbohydrates, with water and oxygen as waste products.
From the point of view of the global ecosystem, photosynthesis is largely responsible for producing and maintaining the oxygen content of the Earth's atmosphere, and supplies all of the organic compounds and most of the energy necessary for life on Earth. It also uses up the carbon dioxide that is produced by members of the animal kingdom, by various processes including exhalation.
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