Your questions asks about the role of a retailer in supply-chain management. The fact of the matter is that retailers are a critical partner in supply-chain management.
To better understand why retailers are so critical to supply-chain management, CSCMP, Esper, Waller (2014) write, "Managing customer and vendor relationships is a critical aspect of managing supply chains. In many cases, the collaborative relationship concept has been considered the essence of supply chain management." Which is to say that without one there is no use for the other, and that the relationship between the two is a vital element of supply-chain management.
Yet, why is this relationship so important? This can be answered in one word. Inventory, which is the raison d'etre of both the retailer and the vendor. This is what joins, unites, and creates the two. Without inventory, neither the retailer or vendor would have anything to sell.
So the role of the retailer in supply-chain management is to manage their inventory in such a manner that the vendor can effectively respond to their need. As CSCMP, Esper, Waller (2014) also write,
"To effectively manage the forward and reverse flows in the supply chain, firms have to deal with upstream supplier exchanges and downstream customer demands. This puts an organization in the position of trying to strike a balance between fulfilling the demands of customers, which is often difficult to forecast with precision or accuracy, and maintaining adequate supply of materials and goods. This balance is often achieved through inventory."
Which is to say that fluctuating product demand necessitates retailers to effectively manage their role by predicting customer demands in such a way that they balance fulfilling those demands with an accurate supply of products. And, inventory is the key to the balance of the reciprocal relationship between retailers and vendors.