Bitcoin keeps track of coins by 'unspent output' (UTXO), not by 'address balance'. There is no such thing as an 'address balance' at the detailed technical level (though you can use the blockchain data to figure out what an 'address balance' might be. Most blockchain explorers display this info according to 'address balance', so it is easy to get confused at first).
When someone sends you some bitcoin, they will create a transaction output that specifies your address as the new owner of those coins. Think of this output (a newly formed UTXO) as a physical coin you have received.
Now, lets pretend you have received 8 transactions, so you have 8 physical coins in your pocket. Those coins may all exist at the same address, but in order to spend them, you must pull all 8 coins out of your pocket. Despite existing at the same address, the coins have not joined together into one coin, they are still individual coins (UTXOs), and thus must be individually specified in your transaction.
This is why that transaction makes a payment by using 8 different UTXOs that all exist at the same address. A transaction must specify each UTXO you want to spend, not each address' balance.