Why do I have to own ETH to send ERC-20 tokens?

It is essential to know that to carry out any sending operation with any Ethereum based token that can be managed from Oxis (0x, Tether, USD Coin, TrueUSD, among others) there must be a few dollar minimum worth of ETH in your wallet; Otherwise, you will not be able to send any of these tokens. 

The reason for this is that tokens such as ERC-20 tokens run on the Ethereum blockchain, so the transaction fee is paid in "Gas", which is the fuel Ethereum uses to perform all operations. More can be learned about Gas here.