Regions are a method of grouping together provinces that are used for a common purpose, so that you don't have a massive list of provinces for each trigger that uses such a thing that has to be updated every time. Naturally, there will be times when a province or subregion is considered part of two different regions, for historical or gameplay purposes. That means that you have to resort to listing at least two different regions, often much more if you're going to have a complex setup like mods like to do, which is much more inconvenient than it would be if custom overlapping regions could be created.
Thanks for the explanation, but I was already aware of what regions are and how do they work [been playing since 2007].
What I meant is I don't see how - overall - the current system is better then the newer one [except with 'overlapping'].
At least from my experience I can attest that the current region map-mode with overlapping sub-regions can be very uncomfortable to use** [then again I don't play with mods so millage might vary].
** conquer/colonize (mission) province from sub-region x --> wait, is x part of region y? or is it z? where does it begin? how many provinces are there? etc.