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Because the task was too big to finish in one sitting and there is no state between started and done. It goes in-progress on day one and stays there, telling nobody anything for a fortnight.
A subtask is a step within a task, with its own owner and status, rolling its progress up to the parent.
Subtasks carry their own owner and estimate. The parent's progress is the completion of its children, so a task at sixty per cent means six of ten steps are finished rather than a feeling.
A build task splits into schema, migration and cutover, with two people on different steps. Each has an owner and a status, and the parent shows real progress without a status meeting.
They will not rescue a task nobody scoped. Splitting work into steps requires knowing the steps; a vague task becomes vague subtasks.
Subtask estimates roll into the parent's, so capacity counts the work once, and logged hours attribute to the specific step.
One level. Deeper nesting is usually a phase in disguise.
Yes, on another subtask or a task elsewhere.
Yes, to whoever owns them.
Fourteen days, every module, no card. Or half an hour with someone who will run it on your own records and tell you where it does not help.