An Activity Diagrams developer designs and creates visual representations of workflows and processes in a system. They use Unified Modeling Language (UML) to depict the dynamic aspects of a system, such as control flow and data flow. These diagrams help in understanding the system's overall behavior, making it easier to spot potential issues and improvements. The developer's role involves defining actions, control flows, and activities, determining their sequence, and clarifying how they interact with each other. They may also be responsible for presenting and explaining the diagrams to stakeholders or other team members.