Wash. Rev. Code § 35.58.010
It is hereby declared to be the public policy of the state of Washington to provide for the people of the populous metropolitan areas in the state the means of obtaining essential services not adequately provided by existing agencies of local government. The growth of urban population and the movement of people into suburban areas has created problems of water pollution abatement, garbage disposal, water supply, transportation, planning, parks and parkways which extend beyond the boundaries of cities, counties and special districts. For reasons of topography, location and movement of population, and land conditions and development, one or more of these problems cannot be adequately met by the individual cities, counties and districts of many metropolitan areas.
It is the purpose of this chapter to enable cities and counties to act jointly to meet these common problems in order that the proper growth and development of the metropolitan areas of the state may be assured and the health and welfare of the people residing therein may be secured.
RCW 35.58.010