As you build a computer system, little things start to show up: maybe that database query is awkward for the feature you are building, or you find your server getting bogged down transferring gigabytes of data in hexadecimal ASCII, or your app translates itself to Japanese on the fly for hundreds of thousands of separate users. These are places where your abstractions are misaligned - your app would be quantitatively better if it had a better DB schema, a way to transfer binary data, or native internationalization for your Japanese users.
A lot of ink is spent on the “monoliths vs. microservices” debate, but the real issue behind this debate is about whether distributed system architecture is worth the developer time and cost overheads. By thinking about the real operational considerations of our systems, we can get some insight into whether we actually need distributed systems for most things. We have all gotten so familiar with virtualization and abstractions between our software and the servers that run it.