When you make a transaction on Ethereum blockchain, you need to pay a network fee (gas fee) to the Ethereum miners. Just as your car can’t run without fuel, your transactions can’t be executed without gas.
So in order to clear your balance out (send all ETH out of a wallet) you need to keep the minimum amount of ETH to cover the fees.
That means the max amount you can send out is = your balance – the network fee(gas fee). It’s highly recommended to keep some ETH in your wallet as you won’t be able to make any other transaction with a zero balance. ETH (Ether) is fuel for your wallet.