Ask HN: If I could add a 100MHz CPU external to a phone, what use would it have?

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?