There is one other case where a structure like a block chain can make sense. It’s an easy way to build an unalterable log. If you give me hash 200000 in the chain I know that log entries 0-200000 are all unaltered.

This is perhaps the simplest easiest use case for a hash based linked list. Of course this is not the complete picture of what cryptocurrencies are; the hash list is just part of the system.