Ecommerce marketing is not one activity. It is a chain: creative that makes someone stop scrolling, an ad or a search result that brings them to the store, a product page that answers their questions, a checkout that does not surprise them, and a reason to come back. An ecommerce marketing agency should be able to work on every link, or at least see all of them.
The weak link sets the result. Strong ads that land on a thin product page pay for visits that leave. A beautiful store with no traffic sells nothing. Good traffic and a good page still lose the order if the shipping cost only appears at the last step. That is why we look at the whole path before recommending where to spend.