For me, strictly immutable and side-effect free programming was a forcing function* to make me really decompose problems into isolated parts. Only when I could do that could I truly say I understood the problem (and its distinct sub-problems) in full.

In that sense, Erlang isn’t really different from any other functional programming except that it also ships with a lot of actor / message passing concurrency dogma which solves a different and equally delightful set of problems (low-bug concurrency) at the same time.

So it’s a double hit of two great ideas in one paradigm.

*Oops, no pun intended.