When people say this, the issue is not that you got 50% off, it is the 50% that you paid for something that you do not really need.
If you happen to be in debt and do not have any money saved, it does not really matter if it is on sale or not. If it was not a need or an investment for your professional or personal development, then you are just lying to yourself.
You did not need it. That is money that could have been used to pay your debts or save for your kids’ education, or to pay off your home, or anything else.