Kornhamn is a historic waterfront spot on the edge of Gamla Stan, where small boats once brought grain in from Lake Malaren. In medieval Stockholm this was part of the working harbour, close to the Baltic trade routes used by Hanseatic merchants.
There is not much to "do" here, but the place makes sense as a pause on a walk through the old city. It helps explain why Stockholm grew where lake traffic, sea traffic, and a defensible island all met.