Answer:
By advancing the clock one hour, the sun appears to set later in the evening. Of course, it also rises one hour later in the morning. The daylight "saved" in the morning is used in the evening. Supposedly, this saves energy, because people will not have to light candles, turn on electricity, or use other forms of illumination at night, or at least not as soon in the evening as otherwise. If this was ever true, it is insignificant today, because of the round-the-clock use of air conditioning and other forms of energy.