1. Court ordered on behalf of a person so someone independent can manage funds either up to a certain age or permanently.
2. Guardians, i.e. parents of a special needs person, sets it up so the special needs person can receive public benefits and still have other expenses cared for. Someone else manages the cash to pay for other expenses not covered by public benefits. Public benefits covers food and shelter and the trust can pay for clothing, furniture, car, gasoline, insurance, etc.
If there is a pool of money, the fiduciary must invest it wisely to get a return for the client according to the Uniform Prudent Investors Act in the California Probate Code. Other income must also be managed appropriately.