What’s the main benefit and what’s the side benefit is really a matter of opinion.
The reason the American public likes and supports DST is that they like having the extra summer daylight in the evenings rather than the mornings. When DST arrives, everyone I talk to is glad they can do stuff outdoors in the evenings; nobody says, “thank goodness we can now start saving a little on our energy consumption.”
In 1986, the beginning of DST was moved up from the last weekend in April to the first weekend in April. In order to justify it under the Commerce Clause, I’m sure there had to be something said about energy. But the reason the change was wanted, and the reason most people are still quite happy with the change (if they still think about it as a change from the old way), is that they’re glad they don’t have to wait until the end of April to start having evening daylight.
So what does the body politic consider to be the ‘real’ reason for DST, and what’s the side benefit, from its perspective? That’s a no-brainer, IMHO.