A discount lowers the price of a single item, but it does not automatically lower total spending, and in practice, sales are just as likely to increase overall spending as to reduce it. Treating a discount as a signal to spend, rather than a chance to save on something already planned, is one of the more common ways a shopping budget quietly erodes.
A markdown changes the perceived cost of an item relative to its own regular price, but it says nothing about whether the item was needed in the first place. Buying three discounted items that were not on a shopping list will always cost more in total than buying zero of them, no matter how large the individual discount on each one is. Sales are also frequently structured to encourage larger baskets, through mechanisms like minimum purchase thresholds for free shipping or tiered discounts that increase with total spend, both of which can push a purchase beyond what was originally intended.
A small amount of planning before a sale begins tends to prevent most of the overspending that happens during one.
That short waiting period is a simple but effective filter, since much of the appeal of an unplanned discounted purchase fades once the immediate urgency of the sale is no longer present.
The clearest way to tell whether a sale actually helped a budget is to compare the total amount spent against what would have been spent buying the same planned items at regular price. If the total is lower, the sale delivered a genuine saving. If the total is higher because of extra items added along the way, the discount on any individual item did not translate into an overall benefit, regardless of how good each individual price looked at the time.
Discounts are most useful as a way to reduce the cost of purchases that were already going to happen, and treating them that way, rather than as a reason to shop in the first place, keeps a sale working in favor of a budget instead of against it.