Conceived by Benjamin Franklin, and first advocated by William Willett it's basically a trick to prolong the amount of daylight we work with to reduce economic costs associated with artificial lighting. Electricity in our day, candles in Ben's time. Satistically it has also been shown to reduce traffic injuries and crime.
Since it's easier to adjust law and concepts rather than people's behaviour it was thought easier to tell everyone that for half the year it was one hour ahead of time. Personally I wish it was always DST since I tend to rise later than most except that it would be darker much longer during winter mornings and the sun would never be at it's highest point at noon.