Because a huge number of cryptos are built on Ethereum’s blockchain (ERC20), they also use ETH to enable a transaction. Unfortunately, transaction fees on the Ethereum network are extremely expensive. In case of high congestion on the network, a €100 transaction can easily cost €10. Of course, you want to limit this as much as possible to keep your profits as high as possible. To solve, among other things, the transaction cost problem, other blockchains like that of Tron (TRC20) and that of Binance Smart Chain (BEP20), support transactions of crypto built on the blockchain of Ethereum. This is called a cross-chain. A cross-chain is the interoperability between two relatively independent blockchains. In other words. It allows blockchains to talk to each other because they are built in a standardized way. For example, it is then possible to send your Tether USDT, which is an ERC20 token, from Binance to KuCoin for e.g. €1.- instead of €10.