I guess the basic thing is this:
You need everyone having the same point of reference/doing the same thing at the same point in time, whether that be for leisure or work/business purposes. It's a society, most of us need to have the same frame of reference, we need to be synchronised (that's why we have clocks and watches in the first place).
If it's decided for various reasons to 'shift that frame' to better utilise daylight hours then you have a choice of two things: you either change the time at which you 'do' everything (complicated) or you simply shift the clock (relatively easy, because the time everything happens stays the same, the time you start work, school, etc).
Factor in the desire to have other countries doing the same thing because of our dealings with them, then changing the clocks twice a year is the best way to do it.