msg.value instead of this.balance

Question

What the theory behind using msg.value instead of this.balance
when e.g withdrawing contributed funds from a contract?

E.g

wallet.transfer(this.balance);
wallet.transfer(msg.value);

Answer 1

msg.value is the amount of ETH sent to a payable public method in a contract.

this.balance is the amount of ETH stored in the contract.