Suppose I could interface a 100 or 200 MHz processor + RAM + storage externally to a high-end smartphone (say via micro USB), and all in a tiny form factor, almost un-noticeable. What useful applications could I run with this weak co-processor? What continuous or periodic tasks can be offloaded to it?
Of course I know that. The question is whether there are applications for which an external, physically separated processor, even if weak, would be useful.
Anything you would want to do with that external processor would be slower and more inconvenient than doing it on the phone processor itself.