Sometimes, a major task has several milestones that makes it look as if the task isn't being worked on, but is in reality being handled. Adding all the sub-tasks under such complex task brings a much-needed structure and clearer picture of how the task is progressing.
Adding a sub-task to an existing task
To add a sub-task to a task:
Head to Tasks, then open the existing task
Tap the Add subtask button, then input a name for it
Click the contact button, locate a teammate you want to assign the sub-task to, then tap Done
Tap Update to wrap up the process