Why do XRP wallets require you to keep a 20 XRP balance?

As you've probably figured, XRP wallets require a 20XRP reserve that can never be moved nor sent. (Roughly $11 NZD)

This has always been a pretty annoying feature of Ripples XRP, but at this point in time, it is for good reason:

If the 20 XRP minimum is not met, any amount under 20 XRP that is sent to the new wallet will not go through and be returned to the sender. However, once the 20 XRP minimum is met, you can then receive any amount of XRP into your wallet.

It is unknown if XRP wallets will have a minimum functionality balance forever, but in the meanwhile, we recommend that you hold onto your private keys for your XRP addresses as the 20 XRP minimum could stay like this, even if XRP moons.