The goal of the roadmap is to give a shared vision about the future development of a product. It is equally important for the teams working on the development of a product, and for the users or customers who would benefit from capabilities added in future.
Having a product roadmap is not a peculiarity of agile product development. A detailed vision of the future of a product has always been regarded as a product roadmap. However, in a traditional, non-agile delivery process organizations are more interested in reaching milestones than in releasing new product increments. In contrast, agility requires frequent releases and flexibility for future developments. Therefore a roadmap is a very high-level vision of future deliverables.
A roadmap is often backed up by a release plan, giving a forecast for the coming quarters of the year. Agility requires that a roadmap should be regularly revised to reflect the changes caused by the external environment and by the progress of development.
The roadmap is not necessarily an aggregate of epics, features or themes. It can be created entirely independently from an existing product backlog. When a product is designed for external customers, the roadmap will probably emphasize what is important for a customer, this way managing expectations. This can be entirely unrelated to the epics used for describing business goals or requirements.