There's a why page in their docs, but basically it's cheaper than a multitasking OS for doing the same thing. The valueprop of Erlang is lightweight threads and message-passing.

My knowledge of microcontrollers is dated, but the frameworks for Arduino etc. seemed limited on their ability to do event or message-based programming; most example apps at least were a polling loop. The classic architecture of setting up interrupt/event handlers and going to sleep was not there.