Critical illness insurance is used to provide a lump sum of cash in the event that you are diagnosed with a life-threatening illness. 2 out of 5 Canadians are expected to develop some form of cancer (that does not include the other illnesses covered)
There are plans that cover just a few of the main illnesses (usually cancer, heart attack, stroke, and coronary artery bypass), and there are plans that cover many more illnesses (including blindness, paralysis, coma, Alzheimer's, loss of limbs, etc.).
In case you decide to get critical illness insurance, but never end making a claim for the benefit payment, there's an option called 'Return of Premium'. You have to decide to get it at the beginning of your policy, and it increases the cost of your policy, but every few years you're able to withdrawal a portion of the premiums you've paid into it.
Every insurance company structures their plans a bit differently, so if critical illness insurance is something you're looking for, it's best to set up a meeting to discuss your family health history, the illnesses you're worried about, and find a plan that suits your budget.