Most brands can tell you their ROAS by channel down to the decimal. Ask them what percentage of revenue each channel actually contributes across the whole customer journey, and you'll get a shrug or a Meta Ads Manager screenshot. That gap is the whole problem. What is channel contribution in ecommerce, really? It's the answer to "how much of my revenue is this channel actually responsible for," not just "what happened on the last click before checkout." Those are very different questions, and mixing them up is how brands end up cutting the exact campaigns that were driving their growth.