Iron.io has 3 functions: the scheduler, the queue, and the processing/worker platform. CRON would just be the scheduler part of that and you'd still be left dealing with where to host workers, how to scale them, manage/monitor, etc.

IronWorker (and IronMQ) automates a lot of this operational overhead and makes it easy to just focus on your worker code.

Also when it scales, you can have hundreds, even thousands, of workers running in parallel.