Why Do Demographers And Geographers Define Regions?

Regions are identified by these experts because 1) the area shares common characteristics that have implications for/affects upon the area of interest and/or 2) the specific areas with unique characteristics have implications for or impact upon the larger area of a region/larger area.  Generally the term "region" will imply either shared or co-dependent/intra-dependent characteristics (cultural, topical, geographical, etc.) that is of significance.