My uneducated feeling is that, in a small society, like a pre-civilisation tribal one where maybe human emotions evolved, this is useful because it helps enact change when and where it's needed.
But that doesn't mean that people want to be angry in general, in the sense that if there's nothing in reality to be angry about then that's even better. But if someone is presented with something to be angry about, then that ship has sailed so the typical reaction is to feel the need to engage.
