On token platforms such as Ethereum, the main "coin" is not a coin like other cryptocurrencies like bitcoin, litecoin, monero, etc. Meaning in this example ether is not a coin, it is instead fuel for smart contracts. So when you want to send a token (that's the smart contract) created on a token platform you have to use that "fuel" to feed into the transfer. Which is also known as "gas".
A simple way to explain it is you need to know the difference between the coins and the tokens. The coins will usually have their own blockchain and you will be paying the tx fee in the same blockchain. The tokens are usually developed on top of any blockchain and when you transfer those tokens from one wallet to another, the fee is detected from the parent chain's and hence you will have to pay the fee in that coin.
This also helps the parent blockchain to grow as that blockchain uses increases, as more tokens are developed on it and more users use that chain and tokens.