Is there such a thing as the world as itself? What can we experience purely as it is by nature? I know, hitting your head hurts and the table you bumped into is obviously real. I'm not aiming for solipsism or claiming that you can make the pain disappear by simply deciding that it doesn't exist.
What I'm claiming is that we can never truly purely experience anything without distorting it with our perception. Even physical pain is subjective. We have different pain thresholds, what is excruciating pain to someone is just a flesh wound to someone else. Even more we distort mental and social phenomena. A person is a friend only because of our perception of her, there is no objective existence as a friend without our perception.
Why care? After all, friends are still friends and a punch in the face still hurts - what changes with this perspective? The key idea here is that by accepting constructivism one opens up new possibilities and sees more clearly the underlying forces that shape one's life. It's no good to fall in despair over bad events. Better just to accept the fact that they're bad due to our perception, due to us not liking that event. That opens up two possible solutions: 1) change the event or 2) change one's perception. Both are useful in various situations. For example, if a stranger insults you and you get annoyed, it's probably easier to change your perception of the insult than the behavior of that stranger.
Another good thing to realize is the fact how much other people affect our perception. And by affecting our perception they also affect our life. Simply knowing that fact will make it much harder for someone to manipulate you. I claim that it's a lot nicer to do one's own decisions and then see who your true friends are, rather than try to do it the other way round.