To ensure that the team builds the right product.

The Product Backlog is a prioritized list of work to be completed by the team.  It is continuously updated by the Product Owner based on feedback from the end-user (as they are exposed to new product increments) and also in response to the evolving technical and business landscape.

It is important that the team provides the Product Owner with sizing estimates for the Product Backlog items to enable the Product Owner to balance business need with the estimated cost of implementation.