And that’s a feature, the reason the map exists.
The phrase reminds us not confuse the diagram or model or overview of the situation with the situation itself. Because they’re not the same.
We make a map so we can leave things out.
By leaving things out, we can help people focus on the core concepts we’re trying to get across. And so, the map of the London subway is not actually the London subway. In fact, it’s not even geographically accurate. That’s okay. The job of the map isn’t to show us precisely where each station is, the job is to make it easier to get around London by showing us a theory of the subway.
And the words someone uses don’t accurately convey everything they’re feeling and thinking. They simply stake out some of that in a way that the speaker hopes will express the point they’re trying to make.
When we decide what to leave out, we’ve made a series of decisions about the story we’re trying to tell with the parts we leave in.