As regular readers might know, I'm working on a book called The Art of Product Ownership to be published by Apress later this year. One of the chapters is entitled "Why have a Product Owner," and a few days ago a bunch of ideas crystallised into what follows...
The aim of the Product Owner is to increase, even maximize, the business value delivered by the team as a whole. The Product Owner does not so much create value themselves as increase the value created by others.
Think of it like this: If the team randomly selected work to do and delivered it to customers, then some value would be created. (For the moment, I'll ignore the scenario where that work detracts from the existing value.) The aim of the PO, though, is to ensure the work done creates more value than a simple random selection. The greater the difference, or delta to use a mathematical term, between random selection and an informed selection the better.