Impressions create brand awareness. Those Amazon tracking ads, showing you stuff you have looked at, might not make you click them but they serve as a reminder that you might want to reconsider buying that roomba. There is also a distinct difference between product and brand advertising. The latter is not conversion oriented. They aim to add their brand to the relevant set for your next purchase decision. So even if you don't click them, you are still being affected. There is even evidence that you don't have to read them, subconscious exposure suffices.