A serious diagnosis brings costs a health plan was never designed to cover: the deductible and coinsurance, yes, but also travel to a specialist centre, childcare, and the income that stops when you or your partner cannot work.
Critical illness insurance pays a single lump sum directly to you when a covered condition is diagnosed. It is not tied to your bills, so it can cover the mortgage as easily as the co-pay. Covered conditions and definitions vary by plan and matter enormously — this is a product where reading the definitions is the whole job, and it is what a licensed agent is for.
As with accident cover, this sits alongside comprehensive health insurance rather than replacing it.