This feature is useful because the new layout support (
ISIS-993) - because of the way it pushes derives facets from the grid layout - can sometimes leave "stale" metadata if the grid is changed.
For example, if an action is moved from a fieldset to some other area such as a column, then the action will appear more than once. That is because the action in its original location causes facets to be derived on the corresponding ObjectAction, and these aren't removed. Meanwhile, when the column to which the action layout is moved is rendered, it doesn't use facets, simply renders what's defined directly the grid.
In the grander scheme of things this sort of thing isn't that big a deal, but it would be nice to invalidate the Isis metamodel (ObjectSpecification) from within the UI to get to a "clean slate".