Round shields and kite shields (the ones with 'pointed bottoms') are often meant for use on horseback. With those designs, there are no corners jabbing into your horse, which is better for everyone.

For infantry, as Christopher Matthew has recently argued in his book on hoplites, round shields have the advantage that no matter how you hold them, they cover the same area. If you tilt a rectangular shield, it will leave parts of you exposed, but a round shield looks the same no matter how you hold it. It is relatively efficient when making a shieldwall formation, as it is wide and doesn't need to overlap and "waste" much surface area in order to form a connected defence. Finally, it allows for more freedom to use spears and other weapons around its edges.

Given that the process for making round and rectangular shields is pretty much the same, there is no particular reason to go for rectangular shields, unless your intention is to create a static shield wall of tower shields (Persian style) or a shield tortoise.