The input-process-output (see ICT model) is a way of looking at the firm's value chain. Supply managers handle the input coming in and the marketing folks handle the output coming out.
global suppliers ->supply chain->[firm-]->marketing->global customers
Supply and Marketing folks are people who sit at either end of the firm and look at the world outside in the firm's value chain. The marketing manager reaches out to customers and supply chain managers reaches out to suppliers and also need to reach "in"to internal firm users.Both off course don't formally talk to one another - organizations are not set up to encourage the talking. Unless there are operations review meetings, that can become mindless and boring. ERP systems can help but ERP speaks to only data and not the gut feel of these important folks.
If you look at the firm as just a processor - focused on creating superior value for its customers-you start realizing how much firms miss out in tapping on to the combined knowledge and expertise of their input and output side teams. More on the ICT and input-process-output.