In a blog post The ‘Zen’ of Program Management Microsoft has stated
Program* managers are the champions of the value proposition.
Microsoft obviously considers distilling a clear and focused value proposition as the main job of a product manager. I couldn't agree more. There's no more frustrating thing to manage than a product without a clearly defined value proposition. And there are very little things which are more rewarding than a product with a clear value proposition. I've experienced plenty of both cases in my career and in some cases both cases were present in the same product. For example, Zemanta Editorial Network has a clear value proposition for advertisers (helping them increase their Google rankings) while we could never really find out what would motivate bloggers to use our services beyond the early adopters base of technology aficionados.
According to Investopedia value proposition is
A business or marketing statement that summarizes why a consumer should buy a product or use a service. This statement should convince a potential consumer that one particular product or service will add more value or better solve a problem than other similar offerings.
The key word in the above definition is why and a product manager should allow himself ample time to think about the problems worth solving (coupled with lots of legwork and hard study, of course). We, the product managers, are problems seekers not solvers! We should get excited when a problem pops up and slightly bored while we wait for engineers to solve it.
*Due to historic reasons, at Microsoft a product manager is called a program manager.