"The more education you have, the fewer jobs you qualify for. They're just better jobs."
The title of this section is a quote from Darrell Whitley, the chair of my department, in a seminar he gave soon after I started my PhD. His point was well-taken: as you gain more education, you tend to become overqualified for most jobs, but the ones you do qualify for tend to be more interesting.
Nobody gets a doctorate unless they like to think and enjoy being challenged; positions that require a PhD tend to depend on exactly those attributes. Jobs that require being able to do a strictly defined task on a short deadline can be better handled by someone with an undergraduate degree; PhDs are better for tasks that require applying knowledge in new ways. The obvious one is university professor, but many large companies - AT&T. Google, IBM, and Microsoft, to name a few - hire PhDs to do research.
Aside from being a requirement for many positions, the process of getting a PhD teaches you how to do independent research; by the time you complete the degree, you should understand how to define your problem and execute a plan to solve it.
Of course, the networking benefits are important as well; when applying for a high-level position, it's always helpful to know experts in the area who can pick up the phone and tell your future employer how wonderful you are!