Why does the transfer function need to have an amount parameter?

Question

The following function must be implemented for a token to be ERC-20 compliant:

    // Transfer the balance from owner's account to another account
15     function transfer(address _to, uint256 _amount) returns (bool success) {
16         if (balances[msg.sender] >= _amount 
17             && _amount > 0
18             && balances[_to] + _amount > balances[_to]) {
19             balances[msg.sender] -= _amount;
20             balances[_to] += _amount;
21             return true;
22         } else {
23             return false;
24         }
25     }

Notice that one of the parameters is the _amount. That's the amount that the user specifies. However, isn't one of the internal properties of a transaction the msg.value parameter? That means that the _amount variable is redundant, and a code smell.

Answer 1

Nope, _amount is not redundant; the variable msg.value denotes the number of wei (not token, but ether itself) sent along with the transaction! See also here.