One good example would be writing a multi-threaded dictionary compression program. Dictionary compression is quite simple, you simply need to encode a bunch of strings to a series of integers and construct a dictionary that can perform the inverse mapping. The main trick is that you need a shared mutable dictionary to store the current list of string to integer mappings.

Doing this with an actor model would be tricky to the point of impossible. Sending a message to the common "dictionary" actor is much, much, much less efficient than just holding the corresponding sharded lock.