It is in the same boat as Ruby was with Rails. One could argue that Rails made Ruby popular but mostly popular as a "web language". Phoenix is a great web framework but Elixir can do so much more by taking advantage of the OTP. I've used Elixir to get up and running with back end services that process data. Using OTP features such as GenStage it becomes easy to handle data as a pipeline. That said, most people who will move to Elixir will probably only use it with web dev and Phoenix.
It's general purpose but well suited for web, soft real time services, distributed data processing, control plain etc. Erlang VM has unique featureset that no other platform really can match so if you need to build HA fault tolerant apps/services it's pretty good match. Coming from a different language one of the nicest things is preemptive scheduler so you don't have to worry about blocking event loop or process consuming all the CPU etc.