Why do you have to pay to use Ethereum?
Every transaction that occurs on the Ethereum network requires computational resources. Miners must be incentivized to process these transactions and create blocks, therefore users pay a small fee per transaction for this.
While some blockchains boast that they are "free" they are simply obfuscating the real cost of the transactions on the network. A "free" blockchain may not charge the user for the transaction fees directly via gas but they do usually require users to pay fees in a variety of indirect ways. Examples of this would include smart contract owners paying the fees and passing them onto the user through the function of the contract, requiring users or dapp owners to lock funds for bandwidth or through inflation by paying a set of block producers.