Yes, this copies the current version of the dashboard to a new one but does not solve the general problem.
In detail:
If you modify graphs on the original dashboard - the change is never propagated to the other dashboards reusing the same graph.
So even with this manual JSON copy you end up maintaining the same graph over all dashboards that are displaying it.
This is often problem in production where you usually want to reuse a graph on tens of dashboards and want graph edits to propagate to each dashboard without manually going and updating them all one by one.