Everytime you make a transaction on ethereum you need to pay a fee to the miner of the block that will calculate the result of your smart contract. While this might change in the future, for the moment fees can only be paid in ether and therefore all users of your tokens need it. Tokens in accounts with a balance smaller than the fee are stuck until the owner can pay for the necessary fee. But in some usecases, you might not want your users to think about ethereum, blockchain or how to obtain ether, so one possible approach would have your coin automatically refill the user balance as soon as it detects the balance is dangerously low.

The creator of the token set it up so the fee is paid in ETH under the hood, users are just not aware of it. Source

Everytime you make a transaction on ethereum you need to pay a fee to the miner of the block that will calculate the result of your smart contract. While this might change in the future, for the moment fees can only be paid in ether and therefore all users of your tokens need it. Tokens in accounts with a balance smaller than the fee are stuck until the owner can pay for the necessary fee. But in some usecases, you might not want your users to think about ethereum, blockchain or how to obtain ether, so one possible approach would have your coin automatically refill the user balance as soon as it detects the balance is dangerously low.

The creator of the token may have set it up so the fee is paid in ETH under the hood, users are just not aware of it. Source