Their purpose is to give a sense of scope, and provide a visualization of the potential reality for stakeholders as a plan of execution. In reality they need constant grooming, and never end up looking like what they looked like in the beginning. The worst part is that they are completely fictional, and inherently full of usually overly-optimistic assumptions.
Instead of using time as a scale for a single desired scenario, we try to cover many possible scenarios in order to be able to visualize many realistic realities for our work which lays ahead. Change and adaptation is built-in from the get-go, which sets much more realistic expectations.
In my opinion roadmaps are an excellent asset which enables a shared understanding of the work which needs to be done, and the dependencies between individual components of said work. The problem occurs with their misuse, meaning they are communicated and understood as a literal plan whose change is perceived as a failure of product management.
Roadmaps are really cool, just use them correctly!