Self interest helps society to achieve its economic goals because people who act in their own self interest (according to the theory of capitalism) will help make their society richer. As they try to make money for themselves, they will do things that help the society as a whole.
For example, we can say that Bill Gates was acting in his own self interest when he invented Microsoft software. When he acted in this way, he developed software and other products that have helped make the whole of society more productive. He has also created a company that has employed thousands of people.
As people pursue their own profits, they invent things that make our whole society richer and which give us better lives. These are some of the economic goals that we have as a society.