Speaking entirely out of inexperience, perhaps it's a situation where you could theoretically do everything with structured concurrency, but you'd have to have

Speaking entirely out of inexperience, perhaps it's a situation where you could theoretically do everything with structured concurrency, but you'd have to have deep understanding of all the details of your system, not to mention needing to update it if aspects change. So it might be less of a hassle in developer time and energy to have a looser model.