In practice
Treat it as a bridge, not a destination: the two outputs drift apart over time and the drift is invisible until rankings move.
If you run it, diff the rendered and served HTML on a schedule so divergence is caught as a bug, not as a penalty.
Why it matters
It was the pragmatic answer for JavaScript sites that crawlers could not process reliably.
Server-side rendering now solves the same problem without maintaining two versions of the truth.