Justification and Rationale
As modern societies operate on the basis of "standard time" rather than solar time, most people's schedules are not governed by the movements of the earth in relation to the sun. For example, work, school and transport schedules will generally begin at exactly the same time at all times of the year regardless of the position of the sun. However, in non-equatorial regions the total number of hours of sunlight in a day will vary a great deal between autumn/winter and spring/summer. As a result, if "standard time" is applied year round, a significant portion of the longer sunlight hours will fall in the early morning while there may still be a significant period of darkness in the evening. Because many people will tend to sleep in the early morning hours, these hours of sunlight are "wasted" for them, whereas if they are shifted to the evening via DST, they can then be "used". People who need to could simply wake up earlier to take advantage of the sunlight then, but this is impractical because of the inflexibility of clock-based schedules.
As days shorten again in autumn/winter, sunrises get later and later, meaning that people could then be waking up and spending a significant portion of their mornings in the dark, so clocks are then returned to the "standard" time. The actual effects of DST can vary significantly by location depending on its latitude and position relative to the centre of its time zone. For example, DST is almost redundant in extremely northern/southern locations because the very long/short days mean that the artificial manipulation of time has little or no practical effect.
Read more about this topic: Daylight Saving Time