Think of the city as its own personal furnace as it can generate its own heat. The concrete buildings and road pavement in city areas will absorb the day's sunlight and radiate that heat out during the night, acting like a natural heater. It usually keeps Seattle anywhere from 3-10 degrees warmer at night than the rural areas.
It can also lead to warmer daytime temperatures -- especially on hot, sunny days. Although in Seattle, the effect is mitigated by nearby water, other cities can be 6-8 degrees hotter on sunny days than their rural counterparts.