The most important thing that Wal-Mart has done with the internet to become the world's largest retailer is to manage their supply chain via the internet. This has been much more important to Wal-Mart than actually selling products over the internet.
Wal-Mart's major selling point is that they are a low-cost retailer. Wal-Mart is able to keep their costs low in part because they work closely with their suppliers. This has allowed Wal-Mart, for example, to keep lower levels of inventory than other retailers. They use the internet to coordinate operations with their suppliers so as to always have enough inventory, but not an excessive amount that will incur things like storage costs.
By managing their supply chain via the internet, Wal-Mart has helped itself become the world's leading retailer.