Choice-supportive bias is the human ability to rationalize the reasons for purchasing an item that is not necessary. Most of us have had the experience of making an impulse buy and then going home and having to explain why you bought it to a parent or a spouse. This is when it is necessary in retrospect to give logical or believable reasons for why you needed something (beyond merely wanting it).
An example might be going to the mall and shopping for necessities and then "falling in love" with a beautiful outfit that you just have to have--it's a great color and it fits wonderfully. Later on then you rationalize that "I needed it for that party that I'm going to, the price was so reasonable, I will wear it for years,etc. to assuage the guilt that might be felt over the impulsive purchase.