By defintion this step is doing:
After the migration of the templates it could happen that a property has the same value on the prototype and on a template.
When a property has the same value in the prototype and in a concrete template, this property is defined as redundant and could be removed.

In this logic we think that a page template is only referenced by one site definition. But this is wrong !
So we can only run this task when we have 1 site definition to migrate.

The task can be triggered by the version of a module if the developer knows that the pages templates of its module are only used by one site def. Then this task is helpful to do a kind of cleanup.