The main purpose of Daylight Saving Time (called "Summer Time" in many places in the world) is to make better use of daylight. We change our clocks during the summer months to move an hour of daylight from the morning to the evening. Today, about 40% of countries worldwide use it to make better use of daylight and to conserve energy. Some countries have different change dates.
Some sources say that the idea of daylight saving was first conceived by Benjamin Franklin. However, the idea voiced by the American inventor and politician in 1784 can hardly be described as fundamental for the development of modern DST. After all, it did not even involve turning the clocks. In a letter to the editor of the Journal of Paris, which was entitled “An Economical Project for Diminishing the Cost of Light”, Franklin simply suggested that Parisians could economize candle usage by getting people out of bed earlier in the morning. What's more: Franklin meant it as a joke.
If you are a fan of the modern-day DST, the credit goes to New Zealand scientist George Vernon Hudson and British builder William Willett.
Although modern DST has only been used for about 100 years, ancient civilizations are known to have engaged in comparable practices thousands of years ago. For example, the Roman water clocks used different scales for different months of the year to adjust the daily schedules to the solar time.