One of the most important tools in a persons career is their ability to work for a living. If that were taken away because of an injury or illness it could seriously impact a persons ability to earn a living. A disability insurance policy could provide a benefit to sustain a families quality of life. It is important to mention that the definition of disablilty in the contract can vary greatly from carrier to carrier.
Typically, something is better than nothing. There are pros and cons to having only employer provided DI. Some like to have a private policy outside their employer plan.