Daylight Map of the World
The decrease in daylight during the winter months is due to Earth's axial tilt of 23 degrees. If the Earth was not tilted on it's axis, the length of day and night would remain steady year long. During winter in the northern hemisphere there is less daylight hours, and the southern hemisphere in turn has longer daylight hours and experiences summer.
The main purpose of Daylight Saving Time (called "Summer Time" in many places in the world) is to make better use of daylight. In Ireland we advance our clocks forward by one hour in Spring to maximise daylight for the summer months. This has the effect of moving an hour of daylight from the morning to the evening. In Winter the clocks are then changed back by one hour to maximise daylight during morning hours.