A subtask is a task in its own right that sits under a parent task. It has its own assignee, its own due date and its own place in reports.

That is the test for whether you want one. If a step needs a different owner or a different date, it is a subtask. If the steps only make sense together and one person is doing all of them, use a checklist instead and keep it as a single task.

Splitting work that did not need splitting is the more common mistake: five subtasks nobody owns separately is five things to chase instead of one.