My take is that the release plan is a mechanism of communicating when the enhancements will be made to the product to the stakeholders and the team.
It is a way of viewing the Product Backlog to forecast when the Product Backlog Items will be completed.
Another technique to forecast feature implementation is the Product Roadmap, Roman Pichler has published a great article on road maps http://www.romanpichler.com/blog/ag...t-roadmap/
A release plan is a tool to show other people in the organisation when you aim to implement the enhancements in to production. Sometimes the customer does not want the change at the end of every Sprint.