It's the only way they can calculate their correct tax liability. The adjustments can be the result of current or previous circumstances. One common reason employers make entries on this line is sick pay. They would enter this adjustment, including fractions of cents involved, on the corresponding section of line 9. Line 9 also corrects errors in Social Security and Medicare taxes reported on an earlier return.
Adjustments pertaining to earlier returns take more work, though.
First of all, they change tax liability. These latter adjustments also require an explanation. Employers can take care of this by filing Form 941C along with Form 941. Make sure the statement is included with Form 941. If these adjustments also mean adjusting an employee's Social Security or Medicare wages or tips for a prior year, there's still a little more work to do.