[ngrilly]

Erlang's fault-tolerance becomes really useful when you wrote a server that manage hundreds of thousands of simultaneous connections (a chat server being the typical example). With Erlang, each connection is managed by its own lightweight process (no callbacks, no promises, etc.). If a lightweight process fails, it doesn't bring down the other processes. Moreover, BEAM can signal other processes about the failed process (Erlang' supervision trees are based on this mechanism).

In a traditional architecture, you would use one thread for each connection (let's ignore the issue of the memory used by each thread), but when one thread fails, it would bring down all connections instead of just the failing one.