From an operational perspective, it's about maintaining knowledge and a centre to undertake processing of that particular visa, especially if it's not automated. If you have a particular subclass where application rates are relatively low because they were always falling into another category, having to maintain that becomes quite high cost. Rationalizing it and shrinking it down into what we term more “streams” of what activity they are undertaking under the banner of the subclass allows that concentration to occur.
On February 1st, 2017. See this statement in context.