Why do I have to fill out the Earlier Year Lump-Sum Social Security Worksheet

You don't have to fill out the Lump-Sum Worksheet, but filling it out might save you some money if you received a lump-sum payment for an earlier year. The payment for an earlier year or years would be shown at the bottom left of your Form SSA-1099. If there's no payment shown for an earlier year, you do not fill out the worksheet.

What version of TurboTax are you using? When you posted your question you selected TurboTax Online Premier as the product you are using, but you would not see the Earlier Year Lump-Sum Social Security Worksheet in TurboTax Online, and you would not be able to fill out the worksheet directly. Are you actually using the CD/Download TurboTax software?

When TurboTax asks whether you received a lump-sum payment for an earlier year, you can answer No even if you did receive a lump-sum payment. Then all of the Social Security benefits that you received in 2018 will be taxed as if the full amount was for 2018, and you would not have to fill out the worksheet. But if you received a lump-sum payment, using the "lump sum election" might save you money. The only way to tell is to fill out the worksheet(s). TurboTax will then determine whether the lump-sum election saves you any money, and it will use it if it does save money. You should use the Step-by-Step interview to enter the information for the earlier year(s). That would be simpler, easier, and less error-prone than trying to fill out the worksheet yourself in forms mode.