Distributors selling to trade customers rather than the public need something a normal shopping cart does not do: different prices for different accounts, credit terms, and repeat ordering that does not make a customer rebuild the same basket every month.
The portal gives each account its own log-in, its own price list and its own order history, with reorder from a previous order in a single step. Stock levels are visible so customers stop calling to ask, and orders arrive in a structured form rather than as free-text emails somebody has to retype.
Where the business also sells to the public, the same catalogue drives a conventional storefront with online payment, so stock and product information are maintained once.