Why and When an android application has multiple tasks?

Question

Activities in an android application arranged in a task back stack.
When we use a single task launch mode for an activity then it can only belong to one task throughout all tasks in the app.
It's vague to me why an application may or should has multiple tasks?
I saw Understand Tasks and Back Stack but it doesn't answer my question.

Answer 1

Take for example the Native Google Dialer. There the Incall Screen is opened in another task. That is how you can be in a call, but at the same time you can browse your contacts. Like almost they are different apps :) Otherwise if the Contacts Activity was in the back stack how would you do that?