The Product Owner needs to be highly available to the Scrum Team. If the Scrum Team has a question about a Product Backlog Item, then the Product Owner should be able to respond within minutes to that question. Responding this quickly ensures that the Team is building the Product in a way that best satisfies the Product Owner. In particular, it helps to avoid surprises about basic aspects of the Product during the Sprint Review Meeting that then lead to wasteful changes or re-work. If the Product Owner does not have this level of availability, it may not cause any immediate problems, particularly if there are other team members who know the business and the product well. However, since Scrum holds the Product Owner accountable for what is built, it is often better for the Team to check its assumptions with the Product Owner.
To learn more about Product Owners, visit the Scrum Team Assessment.