One mistake easy, in fact, almost impossible to avoid when evaluating the return on investment of display advertising is to attribute buying actions to the "last channel" a customer uses when making a purchase.
By that logic, the best channel a retailer has is a cash register at a store or the ordering function of a website. That's the "channel" we can measure as registering a sale. What is difficult, nearly impossible to measure is all the other influences that have lead up to a purchase. Those influences are, by definition, indirect in the sense that all of them, or some of them, have had a rule in moving a particular buyer to purchase activity.
The argument is that prospects often have initial exposure in the research phase that later produces a "buyer." A late-stage visit often gets credited to search marketing, when a user already has made a decision, and is looking for the actual sales channel.
So the temptation is to credit the entire sale to the search channel. That's a mistake of attribution. It can be unknowable how any particular prospect became a buyer, but it likely is the case that multiple information or advertising and promotion channels played a role.