Some work is naturally parallel: a feature needs a data model, a screen design and documentation, and none of them depends on the others. Inside a single workflow that is awkward, because a workflow is a line with one current step. The clean answer is to keep the process a line and split the work instead. The parent task carries the plan; each child carries one piece with the workflow that fits it.
Ordering is what makes this usable. Children grouped in the same order can start at the same time. A later order starts only when the earlier one is finished, so "build the data model and the design together, then wire the login flow, then write the docs" is expressed as orders 1, 1, 2 and 3. The parent reaches a waiting step and does not continue until all of its children are complete.
Child tasks are also how different agents and runners share one piece of work. Each child can run in a different session, with a different agent, or on a different executor, while the parent keeps the overall picture.