[gotstad]

You don't have too, but it is very common to fall into the trap.

If working within a safety-critical industry and wanting to do Agile, typically you'll break down high-level requirements into sw requirements while you are developing, closing/formalizing the requirements just moments before freezing the code and technical file / design documentation.

It's a difficult thing to practice agile in such an industry, because it requires a lot of control over what the team is changing and working on, at all times, but it can be done with great benefits over waterfall as well.