They allow for writing nondeterministic programs which contain various alternatives for the program flow.

The non-determinism is why cache invalidation — and that other hard problem, naming things — are uniquely and intractably hard problems in computer science. Computers can perfectly solve deterministic problems. But they can’t predict when to invalidate a cache because, ultimately, we, the humans who design and build computational processes, can’t agree on when a cache needs to be invalidated.

Cache invalidation is hard because: Everything in life we want to know, changes.Those changes are non-deterministic.