What is the meaning and value of a central purpose?

In "The Objectivist Ethics," Ayn Rand says that "productive work is the central purpose of a rational man's life, the central value that integrates and determines the hierarchy of all his other values." I find that confusing. What constitutes a central purpose? How does it function in a person's life, particularly in relation to other values like a spouse, children, and hobbies? Should I be worried if I don't have a clearly-identified central purpose?