Your roadmap is about direction, while your project plan is about speed.
Roadmap: where are we going and how do we think we’re going to get there. This is primarily driven by business strategy. (see: How to format your roadmap to foster cross-team alignment)
Project plan: when should we get there. This is primarily driven by resources and velocity.
It’s easy to conflate your roadmap with a project plan, but they are fundamentally different tools that are essential to creating your product backlog.