Personal Comments and Observations Only
Disclaimer added.
Since switching servers only becomes a possibility by enabling Multiplayer, the server infrastructure is partly intended to facilitate game management for players seeking to fly together in the same session.
I suspect you’re asking more from a single-player perspective. From what I know, the Content Delivery Network is session and IP aware, and will default you to the nearest Geographic Server to your Region (i.e., USA West, USA East, Asia etc). So you don’t need to have Multiplayer enabled unless you deliberately want to switch servers.
Understand that the FS services are centrally hosted (i.e., Bing Maps, Live WX, Live Traffic etc.) on Azure. Playfab provides session management (i.e., Multiplayer among others) and XBox Live is authentication and provisioning/entitlement (i.e., Login/Logout, what content are you entitled to - Marketplace purchases, FS License type etc.)
All those services are routed to you via Content Delivery Network which aims to find most efficient and bandwidth appropriate route. MS has a CDN but see next paragraph.
As I said in other threads, it not all on Microsoft. National Telcos and your own ISP play a role in determining the quality and speed of the FS services to your end point. There is a large and ongoing interaction between Content Providers like MS and Telcos/ISPs, all who have competing interests in CDN strategy and delivery.
Not an official statement - just what I know of the infrastructure from spending time here. Your Mileage Will Vary.