That extra zero indicates the key pair number derived from the master seed(commonly referred to as address_index).
You can read more about it here https://github.com/bitcoin/bips/blob/master/bip-0044.mediawiki.
The derivation path means:
m’ / purpose’ / coin_type’ / account’ / change / address_index
As you can see the last is address_index which is usually appended at the last of the master derivation path, in your case at
const BIP44_PATH = `m/44'/60'/0'/0`;
This is to generate various accounts (when you click "Generate a New Account" in MetaMask) from the same mnemonic.
So subsequent address paths would be:
m/44'/60'/0'/0/0 //First Account or key-pair
m/44'/60'/0'/0/1 //Second Account or key-pair
m/44'/60'/0'/0/2 //Third Account or key-pair
For the ledger, the first account is usually used for doing transactions, so the address with 0 for the first account is already hard-coded in the string.