Most business plans optimise for normal or best-case scenarios — and break catastrophically when worst-case happens (key customer leaves, founder ill, major economic downturn). Explicit worst-case planning prevents the catastrophic version.
What worst-case planning includes
Plan for losing biggest customer (impact, response, time to replace). Plan for founder unavailability (operational continuity). Plan for revenue dropping 50% (cost reduction sequence). Plan for major product failure (recall protocol). Plan for legal crisis (response strategy).
Why this isn't pessimism
Knowing what you'd do reduces panic when it happens. Plans for worst case are usually never needed but invaluable when they are.