Life insurance will provide money for your family to live on at your death. Disability insurance will also provide money for your family to live on but the insured has not died. Rather, you are still alive, but not earning any income. You still need to eat, wear clothing, and your medical expenses may be significantly higher. In either situation, income has stopped. If the wage earner has died, expenses have also gone down for the family, where if the wage earner is disabled, expenses have likely gone up.
The chances of a disability are much higher than most people realize. There is a one in three chance that a 45 year old will suffer a disability requiring absences from work of at least three months during his working life.