The difference (I believe) is in the stack trace depth, specifically that the trace doesn't die if it happens to go through an async event. You can see in the first gif that if the async box is checked it allows a developer to see the stack trace all the way back to when the form submit event was initiated, despite the fact that an async ajax call was made in the middle of the stack. Normally, the stack trace stops at a response to an async event. You can see this in the beginning of the gif without the async box checked.