Importance of an Activity diagram?

What is the importance of an Activity diagram? Business Analyst Interview Questions
The main use of an activity diagram is to show the work flow of the business and to show the activities happening in the business system and actions completed. In a companies there may be different sections like medical, accounting, HR etc..and each department has different privileges to the system. I.e medical dept can access only medical related screens and HR dept to only HR screens etc..So this activity diagrams shows the activities of different departments with their relation. This will be useful to the developers while designing and coding.