I just posted this on Facebook, and I thought I might as well post it here as well.
This is the marginal utility theory of Daylight Saving Time. If you could, you would allocate your daylight according to marginal utility -- starting with the most valuable hour to have daylight, then the second most valuable hour to have daylight, and so on. Suppose, as seems to true for many people, that your ordering (from most to least valuable) is something like this: The hours you want lit the most are from 7am-5pm. (Don't worry about the ordering of preferences within that period, because you'll get that much daylight even in the dead of winter, at least where I live.) Next, you'd like some daylight in the evening, after 5pm. And least important are the early morning hours, before 7am.
Under standard time, you've got your first period (7am-5pm) covered even in winter (at least where I live, Los Angeles). But as the daylight hours get longer, they are distributed approximately equally on both sides of that time period. This is inconsistent with your preference ordering, because you'd rather get the added daylight on the evening side. By mid-March, you've added a full hour in the morning (6am-7am) and full hour in the evening (5pm-6pm). But you'd much rather have had both hours in the evening (5pm-7pm). Switching to DST accomplishes this. And it does a similar thing the rest of the summer, although the specific hours swapped change.
But in that case, why not have DST year-round? Because if you did, then in the dead of winter you'd have daylight from 8am to 6pm. That means you'd be getting a less-valued hour of daylight, 5pm-6pm, instead of the more-valued hour from 7am-8am.
In other words, the order in which nature provides us with added hours of sun doesn't match our preferred ordering. Nature adds hours in a symmetric fashion, while our preferences order them asymmetrically, wanting to add more in the evening before we add more in the morning.
And in case you're wondering: yes, this scheme does imply that we might want to have Daylight SUPER Saving Time in mid-summer, so that we could trade an hour of daylight at 4am (useless!) for an hour at 9pm (awesome!). But given how much people bitch about changing over twice a year, the adjustment costs of four+ times a year would be too great.