A common topic with many teens is the conflict between their world and the adult world. The teen poet helps define this often abrasive interface. In later years he also gets to see how well he predicted his own mature self. Hold onto anything you wrote when you were a preteen and you will have an even more valuable document of your maturation. When a teen sits down to write, he can of course write anything he thinks, but writing it makes it much more accessible to others.
Reality is possibly the same for everybody, but imagination can be dangerous material.
When you write it, anyone who sees the paper can see your thoughts. You may know, but others may not realize which thoughts are fantasy and which are real experiences or intentions. A teen who journals his thoughts in poetry or any form of writing should be aware that those written thoughts can be misinterpreted. On the other hand, putting thoughts on paper may make them clearer to the writer. Thoughts which come from the unconscious mind continue to grow even for the writer after they are put on paper or on the computer screen.