Protists have their own kingdom because they are neither plant nor animal.
Most protists are unicellular (with a nucleus) and live in aquatic, or moist environments. While most are microscopic, others grow very large such as those which form giant seaweeds. One example of these larger protists is algae. Green, brown and red algae which can be found in the ocean and freshwater produce a great deal of oxygen, and are an important source of food for other organisms.
Terrestrial protists are important for producing oxygen, such as those which act like plants: (Euglenoids, Dinoflagellates, Diatoms, and Algae). These are important to the environment because they can live in the earth and in the bark of trees. Fungus-like protists such as water and slime molds, and downy mildews can be found on decaying foods, plants and trees. Without them (and bacteria), dead objects or unused food would stay around forever.