With many molding processes, many critical properties and dimensions are locked in once tooling has been built. Unforeseen difficulties can lead to large additional expense in tool modification or in costs associated with switching to a different material.
With rotational molding, control over critical part properties and dimensions is retained for the entirety of the tool's working life. At little to no additional cost, adjustments can easily be made to part weight, wall thickness, color, and even material. It allows a luxury of trial and error that other processes simply can't match.