In practice
The spec can be short and it can change. This isn't waterfall. What matters is that the intent exists, stays current and guides the work.
Coding agents made the practice more important. A vague brief produces vague code at high speed.
Why it matters for product builders
Unclear intent becomes rework. That cost is easy to miss because it looks like the build simply took longer.
What changes in AI-first products
The spec now guides production and verification. The same checkable document can brief an agent, then help it test the result.