One reason to use Reroute Dimension is when the original 3D feature has been replaced or modified, so the dimension loses it's associativity to 3D. Reroute Dimension will let you relink the dimension to the new feature.
Another reason could be if a dimension was created off of the wrong geometry; Reroute Dimension will let you correct it.
Sometimes it's easier to just delete and recreate a new dimension. But if the dimension has associated text, tolerance, datum, or positional links, it's much better to use Reroute Dimension.