Hello everyone,

I've been trying to wrap my head around $A.getCallback but the only documentation I could find was not very helpful.
https://developer.salesforce.com/docs/atlas.en-us.lightning.meta/lightning/js_cb_mod_ext_js.htm says:

Use $A.getCallback() to wrap any code that modifies a component outside the normal rerendering lifecycle, such as in a setTimeout() call. The $A.getCallback() call ensures that the framework rerenders the modified component and processes any enqueued actions.

So I made a simple component that displays "Hello World" and a button that fakes an async call using promises and setTimeout. Once the promise resolves I update the attribute of the component and the text changed. I was expecting the component not to be rerendered unless I used $A.getCallback(), but it worked fine without it because component is in scope because of the closure created when helper.handleClick() runs. Here is the code:

Markup
 

<aura:component >
	<aura:attribute name="msg" type="String" default="Hello World"/>
	<c:testCmp2 message="{!v.msg}" />
	<lightning:button variant="brand" label="Submit" onclick="{! c.handleClick }" />
</aura:component>

({
	handleClick : function(component, event, helper) {
		helper.handleClick(component);
	}
})

({
	handleClick : function(component) {
		this.testPromise()
			.then(function(text) {
				component.set('v.msg', text);
			});	
	},
	testPromise : function() {
		return new Promise(function(resolve, reject) {
			setTimeout(function() { 
				resolve('Message Changed') 
			}, 3000);
		});
	}
})

I'm basically trying to learn how things work by breaking them lol. Can anyone show me an example where some async task does not work when not using $A.getCallback() ?

Thanks in advance!