Complex sentence: The Salt Lake City and County Building, usually called the "City-County Building" is the seat of government for Salt Lake City, Utah and once housed the offices for Salt Lake County as well, hence the name.
Split into simpler sentences: The Salt Lake City and County Building, usually called the "City-County Building", is the seat of government for Salt Lake City, Utah. The historic landmark once housed the offices for Salt Lake County government as well, hence the name.