Why do I need a Roadmap?
Usually there are some sponsors in the product. Most likely it's someone from your company's business, or it's some investors. And accordingly, these businessmen or investors need a Roadmap to show that your team is doing the most critical functionality from the list of tasks.
Roadmap is needed so that business or stakeholders (persons interested in the implementation of the project) can understand the timing: what time you are implementing this or that functionality. This is necessary because often, along with the release of certain functionality, it is necessary to plan some parallel additional actions: marketing activities, plan the budget for expanding the team, expanding the staff (for example, sellers). Therefore, the Roadmap is needed for some upper-level deadlines, under which the team subscribed (committed (from Eng. to commit) - means to subscribe to the execution of the transaction on certain conditions, at a certain time).
And of course, the Roadmap is needed by the team itself so that all team members have an understanding of who is doing what and why. So that there is no feeling that you are going blindly and doing some kind of functionality outside the overall picture.