Cooking oil can block plumbing and piping, which allows for waste water/sewage backup into homes and businesses (ew). People assume that flushing used oil down the drain with hot water will work, but drains do not have proper grease management systems, and blockages will persist.
See cooking oil for more information.
Reduce the need for new cooking oil, by filtering it for reuse. Epicurious has a great article on safely reusing cooking/vegetable oil.