A discount can persuade you to spend in order to feel that you saved.

A discount converts spending into a feeling of saving. You did not have the item, you now have it and less money, and the framing has produced a sense of gain out of an expenditure. The mechanism works even when you know exactly how it works.

The usable test is whether you would have bought it at that price without the comparison. If not, you have not saved anything. Deciding what you need before you look at what is reduced is the version of this that actually protects you.