The roadmap is a detailed plan to guide progress toward a goal, a visualization of a strategic plan, and is owned by the portfolio. This schedule shows the intentions of the organization, the tactics needed to fulfil the strategy, but it can also be seen as a sum-up of the organizational purpose (the overall goal). The roadmap can be seen as a promise, which means that we need to be judgmatical in what we are committing to.
The roadmap can be shared with stakeholders, management, customer and the rest of the organization. The roadmap for example supports so that; the business strategy is clearer, the annual plans can be shared with advisory boards and partners and organization-wide goals and the initiatives can be communicated internally within the organization. This means that everybody can easily understand what is coming next and who is responsible and rally behind what the organization shall achieve together.
A road map is different from plans within an initiative. A roadmap defines the high-level goals and is an overview of how to carry them out. A plan for an initiative is more specific and shows the step-by-step actions; what we need to do in order to achieve the initiative. An initiative normally also has a release plan, showing the coming releases, what output to be expected from the initiative and when.