Haven't read the whole thread, so I may be rehashing things, but since I'm a bronze age historian primarily, I'll give it a go.

Chariots are pre-cavalry, for the most part. Their use is depracated, in the sense that they lasted longer than they were entirely useful. There is a substantial chariot culture, and in places like britain, they are used because no extremely good cavalry has displaced them.

In the beginning of chariot usage, the chariot came from the steppe. The newly domesticated horse could not yet be ridden from the forward control position, and as such was not a particularly reliable battle platform.

Using the available materials, attaching a sort of lightweight wagon to the horse was a good way to make a mobile missile platform, from which an archer or javelineer could ride circles round his opponents and also deliver melee troops (nobles, generally) to key points on the battlefield.

Also remember that pre-Greek and pre-Celtic warfare was largely not fought in a 'face-to-face' manner. They didn't charge at each other and hack away with reckless abandon, they generally formed up, exchanged insults and missiles, then one side ran away.

So, in that sense, you have to understand the environment into which the chariot was arriving. Also, once again, remember that it took thousands of years of breeding since the chariot arrived to make the horse suitable for use as a directly mountable vehicle.

More in a bit.