Why do geographers distinguish between dialects and subdialects?

Because they can show the particular environmental differences of the groups.

Explanation:

For people who study geography all the differences in speech are valuable as they determine the various groups, their differences, and the circumstances in which they live, which includes environmental differences.

Subdialects that belong to the same dialect group can have fairly small differences, but these distinctions can point us to the various conditions of life and culture, as well as the history of migrations and connections with different societies. For someone who studies geography, these are all the valuable information.

the minoan civilization flourished in the middle bronze age on the island of crete located in the eastern mediterranean from c. 2000 bce until c. 1500 bce. with their unique art and architecture, and the spread of their ideas through contact with other cultures across the aegean, the minoans made a significant contribution to the development of western european civilization as it is known today. labyrinth-like palace complexes, vivid frescoes depicting scenes such as bull-leaping and processions, fine gold jewelry, elegant stone vases, and pottery with vibrant decorations of marine life are all particular features of minoan crete.  

so basically their unique way of ding arts, as well as architecture.