Normal telephone traffic patterns show that most calling is done between people in a community of interest, in this case a geographical one: the population center.
The host switch provides connectivity between the remote switches and to the larger network, and the host may also directly handle some rare and complex call types (conference calling, for example) that the remote itself is not equipped to handle.
In this mode, only local calling is available anyway, so the billing capability of the host switch is not required.
ESA is increasingly available on digital loop carrier platforms as well as on purpose-built remote switches in order to improve the scope of their utility.
This architecture has become less common as backplane bus speeds and centralized switch fabric capacities have increased.