The Diderot effect is named after French philosopher Denis Diderot who, after receiving an expensive robe, began a purchasing spree because he felt his current possessions felt tawdry in comparison to the robe. The effect consists of two ideas. The first is that we purchase things which feel cohesive to our sense of identity. These items tend to be complementary. The second is that obtaining something that differs from our current possessions can result in a desire to obtain goods which complement the new possession (i.e. are consistent with this new identity). This can result in a spiraling shopping spree.
Moving into an expensive house often leads to the purchase of new furniture and household goods even if our old furniture is fine. We feel a desire to upgrade our furniture in the same way we've upgraded our house.
Marketers often attempt to appeal to a desired identify when advertising their products. For instance, certain brands are associated with an active / health conscious image. We are likely to purchase these items if they appeal to our personal sense of identity.