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.

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