You see... first automobile was invented 1885 by Gottlieb Daimler and Karl Benz, but I can't remember when Rolls-Royce inventors was asked "why would anyone need your automobile? It has four wheels, engine, steering wheel..."
You should know that Joomla! is simply fork of Mambo, and so is Elxis, which makes them fully equal on that matter. Try asking Joomla! developers why would anyone need Joomla! when there is Mambo already.