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Why do I draw Activity Diagrams?

I almost always draw
  1. flowcharts or
  2. activity diagrams
rather than rely on text alone. Remember that visual models are almost always easier to test and interpret than textual models.
As someone once said, "When in doubt, draw a picture."
An Activity Diagram is a type of diagram in the Unified Modeling Language.
Activity diagrams represent the business and operational workflows of a system. An Activity Diagram shows the overall flow of control.
In UML 1.x, an Activity diagram is a variation of the UML State diagram where the "states" represent operations, and the transitions represent the activities that happen when the operation is complete.
The UML 2.0 Activity diagram, while similar looking to the UML 1.x Activity diagram, now has semantics based on Petri nets.
In UML 2.0, the Interaction Overview diagram is based on the Activity diagram.

Activity Diagram

Activity diagrams show flow of control and data flow
  1. Typically used to model
  2. Business process workflow
  3. Flow within a use case
  4. Business rules logic
  5. Functional processes
  6. UI screen flows
Activity diagrams are a technique to describe
  1. procedural logic,
  2. business process, and
  3. work flow.
In many ways, they play a role similar to flowcharts, but the principal difference between them and flowchart notation is that they support parallel behavior.
Activity diagrams have seen some of the biggest changes over the versions of the UML, so they have been significantly extended and altered again for UML 2. In UML 1, activity diagrams were seen as special cases of state diagrams. This caused a lot of problems for people modeling work flows, which activity diagrams are well suited for. In UML 2, that tie was removed.