Used to be for the farmers here but technology took care of that. Now, it's supposed to be for the kids, so it isn't so dark when they walk to school and thus we can avoid remakes of our favorite stints of Carmageddon. Instead, it's dark later in the day so as they walk home from school after this or that after-hours activity, we can play Carmageddon after work - the perfect stress reliever.
Your understanding was mine too (until I wrote a bunch of code relating to building illumination). It's what I learned in school, and seems to be the common sense here in the States.
But farmers are practical people. They have always made the best use of daylight hours, regardless of the nominal time. You could call sunrise 7pm and farmers would still start work then. Daylight saving time isn't for them.
The rest of us, however, are slaves to the clock. Daylight saving time is for us. The time in the winter is actually the correct time. The time in summer is adjusted to allow the sun to provide light further into the evening. Basically, it's to encourage us not to sleep so far past sunrise. This maximizes our wakeful use of sunlit hours, reducing energy spent on illumination and to some extent, heating. There are a string of further benefits to us being active during daylight rather than at night, but this was the initial reason.