A product backlog is a crucial tool that enables the Scrum team to achieve fast, flexible value-delivery flow in the presence of uncertainty. Uncertainty cannot be eliminated from product development. We must assume that a stream of economically important information will be constantly arriving and that we need to organize and manage the work so that this information can be processed in a rapid, cost-effective way while maintaining good flow.
Further Reading:
Book: Kanban: Successful Evolutionary Change for Your Technology Business by David J Anderson
Kanban from the Inside by Mike Burrows