Disability insurance provides for replacement of your individual income during periods in which you cannot earn a living, or you have business expenses which must be paid whether you are at work or not.   

As an example, if you were a doctor or dentist and suddenly had an incapacitating injury, the expenses of the office must continue to be paid.  Salaries, rent, equipment and other vital services of an office must continue to be covered, or the business will shut down.  If the individual returns to work after the illness and cannot start the business again, serious, substantial, long-term loss of income results.  Disability insurance can fill the gap.