If you try using an ability just before GCD ends, the new ability is queued. It is executed as soon as the GCD ends. If you are running a tight rotation, you want to queue abilities as soon as possible, which leads to button mashing.
This addon plays a click every time an ability is queued to let you know that you can stop mashing that key. It works best if your rotation doesn't involve repeating the same ability several times in a row (example: protection paladin rotation).