[kag0]

> Immutable data does not necessarily let you answer the "what state does this value hold" question easier than mutable data.

Yes, it does. You're right that you still have a chain of operations to figure out first. But with mutable data you need to do that, as well as know when you're looking at the data, and what else is running in the system that has access to that data.

It doesn't grant superhuman insight into program state, but it is orders of magnitude simpler than mutable data (perhaps several orders depending on the scope of the data).