During the first two years, Total Disability means the inability to perform your occupation due to illness or injury and that you are not engaged in any other money-making endeavor. After two years, it means the inability to do any occupation with regard to your training and experience.
According to the Council for Disability Awareness, 90% of long-term disability claims are due to illness. On top of that, most claims are not work-related, so they can't be covered by workers' compensation.
Every situation is different but, generally speaking, you want to make sure that your basic monthly expenses would be covered in the event of income loss.