A majority of ‘Cryptocurrencies’ are not native to their own blockchains.
Many new cryptocurrencies, such as USDC, USDT, and PAX, among others, were issued on the Ethereum blockchain. Ethereum is used asERC-20 tokens do not have their own Blockchain, meaning fees are not paid in that token. Instead, ERC-20 tokens exist in Smart Contracts on the Ethereum Blockchain.
To send ERC-20 tokens, a user must inform the Smart Contract to allocate some of the tokens elsewhere. This is done by sending an 'on-chain' transaction to the Smart Contract. Since it is on the Ethereum Blockchain, a user must have ETH available to pay the fee.
This transaction fee, paid in ETH, is known as 'gas.'