What entities get to peek at the data?

That is, to see the data in its full fidelity, with the ability to summarize, store, log, transmit?

If you as a user copy/pasted a complex Excel formula from StackOverflow, should the function author get to see your data?

Obviously not!

If you installed an app, should the app get to see your data in the app?

Of course!

The app is the controller of the data.

Apps must convince the user to trust them with their data before being installed.

But needing to trust the app is also a limiter of apps being installed in the first place.

To be installed an app has to be useful enough for the user to take the leap of faith; to be useful enough it has to glom together enough use cases (plus a viable business model!) to convince a user to install it.

This leads to chunky apps: apps that are larger than they could be, and monolithic, one size fits none.

What if you could make it so authors of code didn't get to see the data?

They wouldn't need to earn the trust of the user to run, because they can't do anything with the data like transmit it back.

If a tree falls in the woods and there's no one there to hear it, does it matter if it makes a sound?

This would allow much smaller bits of code to be viable.

Much cheaper to produce.

In fact, they might be so cheap that only the energy of hobbyists and tinkerers is necessary.