This is a great question. I would say that we need to rationalize and explain away our purchases for two reasons.
First, we live in a consumer culture. We know that we consume too much and we often buy things that we do not need. So, we need to rationalize buying things that we have no need for. Moreover, because this pattern has continued for so long, we have a closet full on things that we will never use.
Second, since the economy is not great, we feel worse in spending money. Too often we buy on credit. This is part of the problem. In view of this poor spending, we need to justify our purchases. There is a sense of guilt.
There is an inherent self-criticism when we succumb to non-rational appeals of advertising. For example, if we buy a car that is non-practical because a sexy girl appealed to us in an ad, we are admitting that we are susceptible to that kind of emotional appeal. When our friends question our judgment, we rationalize the purchase by citing gas mileage, etc. The human condition, from a social Darwinian perspective, does not favor non-rational decision-making, so we "construct a reason" (rationalize) that was not really the motivation to purchase, to disguise this perceived weakness.
I think that's because we value money that much. Purchases entail money. And if we can't justify how we spend our money reasonably, we'll just end up bothered to death. We have to assure ourselves that we use each single penny wisely.