It was first implemented in Germany to conserve energy during World War One by aligning people's active hours better with the daylight hours. By the end of World War One, almost every country involved had implemented it, though most repealed it after the war eased rationing needs. It was then reimplemented almost worldwide during World War Two rationing, and re-repealed again afterwards. The modern DST practice mostly dates to the oil shocks of the 1970s, though the U.S. expanded it in 2005. Simplifying a lot here, but urban areas usually want DST, since they're more likely to schedule their daily lives by the official time, and rural areas generally don't want it, since they usually schedule their daily lives around daylight, and the disruption of DST can be a significant hassle. In fact, prior to the 1970s, official time was set locally, and almost every city used DST while most rural areas didn't. The rise of AC means that DST doesn't save much energy anymore (the hours that people are awake are now hotter on average, so more AC is used even as less lighting is used), but it has one rather major impact: by aligning our commutes with daylight better, DST reduces traffic fatalities by a significant amount, with some studies pegging the number as high as 10% for pedestrians.
As for breaking down other things, I've long been in favor of 10 day weeks, with 3 per month, then a 5 or 6 day fill period to complete the year. Days 1, 6, and 10 would be "weekends," so a workweek would be 4 on, 1 off, 3 on, 2 off. This would make 70% of days weekdays, compared to 71.4% now, not including the fill period. Of course, the truth us, nothing is ever going to drastically change - the thorough globalization of markets means that we're pretty much stuck with the basics of what we have. (Fun fact: after World War One, the U.K. kept DST, so even when the U.S. repealed it in 1919, New York City alone kept it just to keep the relative trading hours between the NYSE and the London Stock Exchange constant.)