You need to pay taxes on all your income. If you have not paid taxes on your income then it can be hard to spend the money domestically without the tax office finding out. To get around this people do take their cash abroad in order to spend it there. This way the tax auditors have a much harder time tracking their expenses and they can get away with tax fraud. And that is why you now have to declare any larger sums of cash when going abroad. They have to check that you paid your taxes.
The government knows all your bank transfers so if you do a crime and transfer the crime money by bank transfer, they can see where it went. But if you just withdraw the money and take it to the next person, they can’t see it. So they put checkpoints in certain places (border crossings) where you have to declare the money or else they steal it from you. This gives them a clue where the cash money is going.
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