The Center on Budget and Policy Priorities (CBPP) has a report up on 4 Reasons Why Disability Insurance Is Especially Important to Less-Educated Workers.
Let me be less diplomatic than the folks at CBPP. Less-educated workers are frequently people with lower cognitive abilities. We do not live in Lake Woebegone. All the children are not above average. Some are born with lower cognitive abilities. The cognitive abilities of others are permanently stunted by difficult childhood circumstances. Lower cognitive abilities lead to lower educational achievements. Adult education is of only limited use for people with low cognitive abilities. They lack the ability to profit from it. People with lower cognitive ability are at a huge disadvantage when they develop medical or psychiatric problems. All they are suited to do is to work at jobs with low skill requirements and those jobs aren't in offices. Those jobs generally involve significant exertional requirements and offer limited tolerance for psychiatric issues. If all you ever had to offer an employer was a strong back and a good attitude you're in big trouble if your back loses its strength or your good attitude isn't so good.