In practice
It communicates sequence and intent, not a delivery schedule. Many roadmaps use now, next and later to show order without promising an exact date.
A roadmap isn't a backlog, which stores everything you might do. It also isn't a release plan, which commits scoped work to dates.
Why it matters for product builders
A roadmap should answer “why not my thing yet?” without a meeting. If it can't explain the order, it is a slide rather than a working tool.