We've all been in the situation where either we have been told (reproachfully) that we "don't know anything" about the situation - or where we have ourselves have committed this accusation.
It is commonly said in heated situations (and we're not all hotheaded or confrontational enough to personally experience it), but that doesn't mean there isn't a relevant lesson for everyone.
Why is this such a well-known social situation? Because judgments are made too early - and this happens when people don't have a full grasp of situations. While this isn't difficult to extrapolate, let's take it a step further... not having grasp of the situation occurs because of our assumptions, overestimation of our understanding, and lack of communication.
Everyone's life is as complex as yours - and to try to fix/understand/help/affect another's can be equally complex (especially in the heated situations when this phrase is used).