Why does the ERC20 Token wiki example subtract the balance of address(0) to calculate total supply?

Question

When reading the ERC20 Token Standard in the Wiki, I've noticed that in its Sample Fixed Supply Token Contract, the value of balances[address(0)] is subtracted from the contract's total supply to calculate the total supply.

Here's what totalSupply() looks like at the time of writing this question:

function totalSupply() public view returns (uint) {
  return _totalSupply.sub(balances[address(0)]);
}

To give more context, _totalSupply is created when the contract is instantiated and .sub() is a library function from SafeMath:

contract FixedSupplyToken is ERC20Token {
  using SafeMath for uint;

  mapping(address => uint) balances;
  uint _totalSupply;

  constructor() public {
    decimals = 18;
    _totalSupply = 1000000 * 10**uint(decimals);
    balances[owner] = _totalSupply;
  } 
}

Now there are two things to me that are unclear:

1. Considering that at the time of creating the contract, only balances[owner] should have the value of _totalSupply, why are we interested in balances[address(0)] in the first place?
2. Assuming that there's zero value in balances[address(0)] anyways, why is totalSupply() subtracting it every time from _totalSupply?

Answer 1

Sending tokens to address(0) is a means of burning tokens.

When calculating the total supply this way, tokens can be removed from existence without ever having to update the internal _tokenSupply field. , which could potentially introduce harmful bugs.

Using ERC-20's transfer function, you can send tokens to any address, including address(0). This contract considers transfering tokens to address(0) to be the same thing as burning tokens. That's because as the balance of address(0) increases, the value returned by the function totalSupply decreases. To the outside world then, the tokens that were sent to address(0) no longer exist.