That derives logically from the way Commodore implemented disks. If you bought a 1540 or 1541 (or any other Commodore drive) for a C-64 or VIC-20, it had an onboard 6502 to run the disk drive. The interaction between the computer and the disk drive was somewhat similar in concept to fetching a file from a network server.

This could be useful to save on costs in computer labs... my grade school used multiplexer boxes to share a single 1541 across four C-64's.

128D had 3 cpus :)

I wrote code to do this between a C64 and a 1541 disk drive when I was in high school. It got me to the international science fair and (probably) earned me a full tuition scholarship for undergrad.