Celluloid for Ruby implements the actor concurrency model.
It's not exactly the same, but running on JRuby it's a reasonable platform for writing concurrent applications in a highly productive language and style.
Elixir is thus able to execute Erlang code directly including OTP. This means you can use OTP as you would in Erlang but without the somewhat arcane syntax. Thus it can be said, you can use the "real" thing.
However it is very useful to have access to actor style concurrency outside of the BEAM ecosystem.
One of the really nice things about Celluloid is you can start up the actor system inside an existing non-threaded and non-actor aware system and write concurrent code.