Being an avid user of Apple's products, one of the standout features that I enjoy is the on-device processing of my personal data. Be it searching for an image, or Siri suggesting a calendar event based on a flight in my email, it's extremely useful.
However, I do notice the toll that it takes on my devices as each has to index and process the information. This is where distributed compute could be a winner.
If my iPhone, MacBook, iPad & Apple TV could work in unisen to offload heavy tasks to processors not in use, it could speed up & even improve the battery life of the device I'm working on.
I can also imagine it being userful in acting as an offline iCloud sync. When my devices can connect to eachother, but not iCloud, to replicate data between themselves to reduce the potential for loss & improve my productivity.
There's something quite nice about starting a file on one device & it being there, up to date, on another.