We use product owners for clarification and as a sometimes as a conduit to the end users. When the story cards or use cases seem a bit nebulous, the product owner is charged with answering our questions and ferreting out the true meaning of the request. This is particularly useful during the sizing process.

We also do value sizing in addition to effort sizing and the PO is responsible for guiding that process. They keep track of the ebb & flow of the value ratings for each of the stories so that the priorities can be re-assessed for each sprint.

And as John said, the PO can assist in problem resolution as well. In a consulting gig, if the ScrumMaster is from the vendor, sometimes it's more advantageous to have the company insider run interference than trying to navigate the politics ourselves.