Easy, write down a list of individual features and order them, job done? If only that was the case!
A product backlog is not simply all the product features, it is all the work needed to build and deliver the product. This includes all features, functions, enhancements, and fixes, all that boring engineering work, infrastructure commissioning, documentation and loads more. It is also a planning tool, letting us know what is most important to the business an gives us some indication, along with velocity, when the project is expected to complete.