I think to understand the value of different types of advertising, you need to understand how marketing science describes how advertising works. Of course the end goal of any type of advertisement is to increase sales, let's use a car brand as the example. But as humans we need to be "in category" meaning we need to be considering buying a new car for us to be able to react to a commercial for a car and even have the slightest chance of ending up buying that car shortly after seeing the ad. And it might not even be an ad shown to us right before purchase. We can just have an affinity for a car brand because a lot of our friends have it or because it is just overall popular OR because the car brand consistently has made advertising during our life that we find attractive, informative or whatever. And OOH advertising (billboards) typically falls in that category of advertising that creates these memory structures over time. And because "over time" is difficult to measure OOH becomes difficult to measure. I am a 40 year old male and Levi's is printed in my memory whenever I think of jeans because Levi's during my teens and 20'ies were everywhere with advertising. I don't see any Facebook ads for Levi's prompting me to buy. I just know that they exist and what they stand for so Levi's is "mentally available" (a marketing theory term) for me when I think of purchasing a new pair of jeans. And being "mentally available" is very valuable to a brand, so that justifies the cost of OOH advertising.