GeoDNS is a technology that allows you to point web users to a nearby server. That's great, but you still need to coordinate each server. For instance, if you have distributed app servers and a single database server, you still have long distance communication/slowness whenever you need to run database queries.
One solution to this problem that Fly provides app-level support for is to allow you to have a fast local read-only copy of your database on/near each server, so that only inserts/updates need to run remotely. Your regional copies basically fail whenever there's an update, and Fly's platform re-runs the request on the leading webserver (which replicates the update to the regional database servers).
How well does it work? Never used it. But it would save you the trouble of having to notice the problem, come up with a solution and then implement it.
One solution to this problem that Fly provides app-level support for is to allow you to have a fast local read-only copy of your database on/near each server, so that only inserts/updates need to run remotely. Your regional copies basically fail whenever there's an update, and Fly's platform re-runs the request on the leading webserver (which replicates the update to the regional database servers).
How well does it work? Never used it. But it would save you the trouble of having to notice the problem, come up with a solution and then implement it.