I know that an activity diagram is:
compared to a flow chart.
But what are the possible reasons for using an activity diagram instead of a flow chart apart from those mentioned above?
An activity diagram is not simpler than a flow chart. A flow chart is a simpler (earlier) version of an Activity Diagram.
Flow charts were invented at least 20 years earlier and are commonly used by non-programmers to document workflows. Any business process where people have to make decisions can be described as a flow chart. So, for example, a clerk making decisions about how to file complicated information might have a flow chart on the wall.
Because a flow chart is a subset of an activity diagram, there's actually no decision to make. A flow chart naturally grows into an activity diagram if you need to add more complex descriptions.
Sparx have a good explanation of the range of things you can describe in an Activity Diagram