A manual task defines a task that is external to the BPM engine. It is used to model work that is done by somebody who the engine does not need to know of and there is no known system or UI interface. For the engine, a manual task is handled as a pass-through activity, automatically continuing the process at the moment the process execution arrives at it.
Manual tasks have no real benefit for automating processes. It exists to provide insights into the tasks that are performed outside of the process engine.
A manual task:
<bpmn:manualTask id="manual-task" name="Manual task" />