Why is an electrical shock delivered to correct abnormal heart rhythm?

Highly effective: It "sets the clock back to zero" so to speak. It synchronizes all the electrically conducting cells in the heart to begin their cycle at the same instant. It interrupts and stops abnormal rhythms. It doesn't hurt and there are generally no side effects so in that regard, it's better than medicines. The downside is that one must be put to sleep, albeit briefly, for it.