The activity diagram is important because it gives an outline of the work flow within the business as well as the activities and action completed.

For instance with a company, there is likely to be more than one department. In such a case each department, will have different access levels to the system.

So if there is a Medical, HR and Accounts team they will only have access to screens that relate to each. The activity diagram will highlight the differences within the departments which will be very helpful for developers when they are designing and coding.