What do "regions" do?

From the May 26th Dev Diary:

First, regions isn't something that is going to affect you directly but it works somewhat like how it have done in the Europa Universalis Games. It's an area on the map that denotes a region with a name and it's mostly used to improve on our localization of things, such as hunting for tigers in India or hunting a deer in western Europe. So no longer will you find Tigers in the woods of Poland if you manage to move your capital of your Indian Empire out of the subcontinent. You can see these regions by opening up a province and click on the new region icon to get an outline of the region. It's also possible to search for regions in the old title finder.

In short, it is a modding and event feature, allowing to know where in the world an event takes place. This is not something you, as a player, will interact with (unless a decision requires you to control all region, for example)