If one income source stopped tomorrow, what would happen to your household? For a lot of people, the honest answer is “everything changes immediately” — and that's a diversification problem, not just a savings problem.
Diversification isn't only an investing term. Relying on a single income source concentrates your household's risk the same way holding one stock does.
This doesn't mean everyone needs a side hustle. It can mean a working partner's income, a small secondary income stream, or simply an emergency fund large enough to buy time while you find a replacement. The point isn't to eliminate the risk — it's to know how exposed you are, on purpose.
The Financial System Check's diversification pillar is really asking one question: if this income stopped, how much room would you have to react?