why do geographers divide areas into regions?

Answer 1

In simple terms, an area is a large and heterogeneous. To study one is difficult. If this area is divided into 'regions' which are homogeneous the study becomes far easier. The regions can be on the basis terrain (as suggested in answer 1, above) or land use, or distance from the sea, climate etc. By dividing into regions, causal factors can be studied as can interrelationships.
Regions can be large or small.
So, in sum Area - hetrogeneous. Region homogeneous

Answer 2

A lot depends on the natural terrain of the area