User acceptance testing answers the question can businesses create the business outcomes they are looking for efficiently with the solution.
The system may work exactly as the requirements specified but not pass UAT. In this case, the system testing passed but the requirements were not correct. The traceability of the requirements back to the business outcomes or business requirements was not fully thought out.
If something fails UAT it does not necessarily mean that something was done wrong. Sometimes a business needs to actually use the system to see if it fulfills their needs. A new car works but may not fit your use of a car.
The focus of the business analyst is on the value the business receives from the change. The better this value is understood the smoother UAT will be.