The backstory starts in the pre-internet age when sales folks were very reluctant to adopt technology because they wanted to keep their leads to themselves. After all, commissions were earned depending on who “closed” the sale. Similarly in direct mail and catalogs, the coupon code and a mailed in order identified which mailing worked. As digital and internet marketing developed, the legacy metrics were adopted and “last click attribution” became important. If you think about it “last click” or the ad that you clicked mirrored the “sales closing” and fitted neatly with the old direct mail coupon code model. All you had to do was to put a code on the ad, email and upon checkout just match the ad to the transaction and “Eureka” you had attributed the ad spend, specific email or ad to the purchase.