Why? Are we really saving time? No, we're "saving" light!
"Daylight saving time (DST)—also "summer time" in British English—is the practice of temporarily advancing clocks so that afternoons have more daylight and mornings have less. Typically clocks are adjusted forward one hour near the start of spring and are adjusted backward in autumn. Modern DST was first proposed in 1895 by George Vernon Hudson.Many countries have used it since then; details vary by location and change occasionally.
"Adding daylight to afternoons benefits retailing, sports, and other activities that exploit sunlight after working hours, but causes problems for farming, evening entertainment and other occupations tied to the sun...
"Its effect on health and crime is less clear. Although an early goal of DST was to reduce evening usage of incandescent lighting, formerly a primary use of electricity, modern heating and cooling usage patterns differ greatly, and research about how DST currently affects energy use is limited or contradictory.
"DST's occasional clock shifts present other challenges. They complicate timekeeping, and can disrupt meetings, travel, billing, recordkeeping, medical devices, heavy equipment, and sleep patterns." (link)
You can read a brief history of Daylight Savings Time at this web page - Benjamin Franklin first wrote about the idea in 1784 in an essay titled "An Economical Project", when he was living in Paris
If you want to learn about Daylight Savings Time in every country of the world, look at this Wikipedia article.