When we wash our plates, the thin film of grease melts and goes down the drain. When the grease contacts with the colder piping downstream the grease solidifies and coats the inside of the pipe with a thin film . After years of use this the film builds to a drain stopping mess that can only be cleaned by professional drain cleaning equipment. A pressure reducing valve reduces the pressure of the water in the pipes of your home so not to cause undo wear or void warranties on you washing machine or dish washer. Pressure reducers also save water by lowering the pressure.
Less water flows through faucets showers etc.
A P-Trap sometimes called a Stink trap prevents sewer gases and other forms of vermin from spewing from the drain. Rust dissolves into the water giving it a brownish color. The only cure for this is to replace the worn out piping. Sometimes you may experience brown water from work that has been done on the water system and dirt has entered the system this usually can be flushed by running the water for a few minutes. Water hammer is the bang you hear when the water is shut off very quickly.