That's just like how shampoo companies advise you to "lather, rinse, repeat". It's a great way to boost sales, but besides being unnecessary it's terrible for your hair, your water bill, fish living downstream, and your wallet.
It’s no accident that they code a loop with no termination condition (except the implicit emptying of the bottle). Really boosts sales!
It just feels cleaner. I assume it's because the additives in the shampoo become overloaded on the first use, and are active on the second.