When we reach for it
Every engagement with an interface. Deciding in a design file is far cheaper than deciding in a pull request. Reviewing in the browser on a real device with real content is still worth more than any number of exported images, so the file is the start of the argument rather than the end of it.
When we would argue against it
Treating the file as the deliverable. A design that has diverged from the shipped component is a document that misleads everyone who opens it. A file also cannot answer what happens on a slow connection, at 200% zoom, or with a name three times longer than the placeholder. Those get decided in code.
What it looks like in delivery
Tokens and components mirroring the code library, and the file updated when the code changes rather than only before. Tokens defined once and used in both the file and the stylesheet, so a colour that fails a contrast requirement fails in one place and is fixed in one place.