Accounting, transaction reversibility, and taxation.
In real life if you want to give somebody a dime, you just do it. But online you have to query your account to make sure you have enough money to make the payment, reserve the funds so the payment can go through without stopping other pending transactions, pass it the amount through some infrastructure where you can dispute the charge at a later date (if needed), and you also have to make sure the government is able to track your transactions for tax and anti-money laundering purposes.
For Amazon it's a different situation because they're charging you for a service based on usage. They have given you an account number and they handle all the accounting on their end. It's not a general payments system.
In real life if you want to give somebody a dime, you just do it. But online you have to query your account to make sure you have enough money to make the payment, reserve the funds so the payment can go through without stopping other pending transactions, pass it the amount through some infrastructure where you can dispute the charge at a later date (if needed), and you also have to make sure the government is able to track your transactions for tax and anti-money laundering purposes.
For Amazon it's a different situation because they're charging you for a service based on usage. They have given you an account number and they handle all the accounting on their end. It's not a general payments system.