A mechanical filter (meaning physical filter fibers obstruct contaminants, as opposed to adsorption filters, which use chemical processes) which traps large particles suspended in water.  Water with high sediment levels, typical in very old municipal water systems such as those in New York City, may rapidly clog fine filters and shorten their life, reduce water flow, and generate bad tastes in the water.  Sediment filters trap those larger particles before the water reaches other filter elements, improving filter performance and extending life.