Many languages have a ‘killer app’ that serves as an important gateway into the language - they’re often the reason why people adopt those languages because it gives them a clear reason to use them and thus becomes an easy (and arguably, important) route into them.
As an example, Ruby’s biggest gateway and killer app is Rails, Elixir’s is Phoenix, Python’s was Django (in the early days at least), Swift’s is iOS app development, Dart’s is Flutter… you get the idea 
If you ask yourself where those languages would be without those applications, you begin to get an idea of how useful and beneficial they can be.
So, what do you think is Erlang’s main gateway application - and if you don’t think one currently exists, what would you like it to be?
I’ve posted a similar thread [here] for other BEAM languages - as that would make for an interesting thread in itself.