My cashier gave a regular ten percent off and the POS asked her for a reason. Why does it do that?

Because a discount without a reason is money nobody can explain later. The Discount button always asks why, and the reason is stored on the sale itself rather than lost in conversation. A cashier can see what a discount would do, but a manager has to type their own four numbers to apply it. At the end of the shift the Z report shows the total given away, and each sale names the manager who approved it. The Staff, PINs and hours tab in the docs explains what a manager approves.