Let’s say you built a web app and it became globally popular. You’ve got users scattered across North America, Europe, and Asia.
In theory, you could shard your data globally: build 3 sets of SQL Servers, one on each continent, and split up their data.
In reality, this usually creates more problems than it solves.
Users are mostly active in their own time zones. So your North American database servers will be getting hammered while your European and Asian database servers are sitting around bored. That’s a great thing if you need to plan database maintenance, but it’s a terrible