State management question

[learning]

So I am using the example on the documentation.. I am assuming

onLeave (client) {
    delete this.state.players[ client.sessionId ];
}

is supposed to trigger a change.operator delete event on the client side but nothing happens. on message and onjoin seem to work as intended.

I am using

arena.listen("players/:id/:attribute", (change) => { })

which makes a lot of sense but what would be the use case of

arena.onStateChange.add((state) => { })

?