I always thought that it was another piece of the puzzle being completed-for the good guys. The fact that the Godslayer, gave life to a would be Gods horse seemed rather fitting. Special horse for a special boy.
The fact that Garion also used his power to do something that B & P would not have attempted also spoke of his growing up, and the seemingly impossible act whilst foolish in P's eyes, was also an indication of Garions huge capabilities as a sorcerer, gaining him some grudging respect from his peers.
I liked the fact that it knocked Pol and Belgarath a bit off kilter, as at some point they needed to let him learn his own way, and not assume to know everything about him and his abilities.