Append-only b-tree/skiplist/whatever immutability can be combined with "checkpoint hashes" (think git) using for example SHA512, that can be transmitted elsewhere at a low cost and used later to validate system integrity. Any data store tampering would be detectable. In a mutable database you'd need to transmit whole transaction log for the same integrity guarantee.

Thus immutable store makes it trivial to implement systems that need to have full history and audit trail.

When there's only a trivial amount of data involved, immutable state can solve a lot of headaches. No programming error can lose or corrupt data for instance - every case can be traced back after the fact.

I hope some immutable store databases take off that cover those use cases. Make it a graph database, bonus points if the underlying engine implements directed hypergraph. And artificial intelligence reasoning engine. :)