When I first learned about Active Directory sites, the concept was explained to me as being locations of readily-available connectivity.
There's an easy analogy for it: islands. In island states, people live on islands, but not everything people need might be available on their island. Additionally, something on their island might break and there are only a few trade routes for goods and services.
In this analogy, the trade routes between geographical locations are the networking connections between Active Directory sites, the islands of readily-available connectivity. The island's roads are that readily-available connectivity: you can use them all you want, without additional cost.