What are the proper circumstances to use immutable facts? Anytime you need an undo or audit features?
Immutable facts are great if you want to:
1) Avoid a large amount of updates/race conditions on your tables. Immutable facts, being immutable, can be done using purely inserts.
2) Want to keep a trail or context to the data you're storing. This has a benefit of keeping a log that you can later use to trace issues, behaviours or generate statistics. If you're keeping a state in a database column, without external logs you aren't able to check when or why that state changes. Immutable facts keep track of this for you, but require quite a bit more overhead.
I would almost prefer some way to "ghost" a row where without a specific switch the DBMS will never return it from a query.