An activity diagram is one way of modeling the all-important dependencies and sequences that surface when an operation has multiple goals.
An activity diagram is consequently desirable for modeling the type of coordination required where a process cannot be continued until certain conditions are met (for example, new data is available). So, states in such diagrams now represent expression execution or action execution, and the focus is on triggering a transition between the states of such a system, and thus synchronizing and coordinating different actions.