A single A record sends everyone to the same endpoint. GeoDNS lets the answer depend on where the lookup appears to come from. On Cloud DNS that can be coordinates, continent, country, ASN, IP or subnet.
You see the resolver, not the user
Authoritative DNS sees the recursive resolver. That is usually close to the user, not identical. Design fallbacks for resolvers that sit far from their users (some enterprise and public resolvers).
Finer than country
Country maps are a start. ASN and subnet splits matter when a market shares a country with a different network path, or when you are canarying a carrier.
Health has to ride along
Steering to a nearby region that is down is worse than a farther region that works. Attach health checks so unhealthy endpoints leave the eligible set.
This is not a residency control
GeoDNS can send people toward in-region endpoints. It is not a legal guarantee. The application still has to enforce tenancy and data residency.