Why is it important to understand the original meaning of a piece of literature before you try to apply it to your life?
We need to talk first about what we mean when we refer to "the original meaning" of a literary work. Unless we have access to the author's thoughts on what the original meaning was, for example, in an interview, and we can assume the author is accurate, we are always interpreting the meaning of the text. The author writes a story. Each of us brings to that story our own thoughts and feelings, and meaning is made for us in the intersection of those words on the page and what we bring. Authors are often quite surprised by how their works are interpreted, and I have provided a link to an article in which some great authors have commented on this. So, you see, all of this makes the term "original meaning" problematic.
Now let's suppose that you misunderstand the original meaning of a literary text. The example provided in the first response is a good example of this. In a situation like this, a lack of understanding of the author's message, an original meaning, can lead to some unusual and unintended consequences. A more extreme example might be Swift's "A Modest Proposal," which suggests that killing and eating babies will solve a country's hunger problem. I hope that no one who misunderstands Swift's message, which was to call attention to Ireland's plight, ever chooses to act upon it in real life. Another example, from the Bible, comes to mind, the idea of "an eye for an eye." This was meant to limit the quality and quantity of justice, so the justice or compensation was not excessive, so that it was in keeping with the nature of the crime or tort, not to encourage people to put out one another's eyes.
Aside from these more unusual situations, though, acting on the original meaning of a literary text in one's own life is a much more complex issue. We need to think about what in literature causes us to act in a particular way in our own lives and whether or not it actually causes this or simply reinforces what we have already brought to the page. If I read a novel about a passionate and adulterous relationship and I decide to leave my husband, it probably does not matter very much what the author's original meaning was, since this is something I cannot truly blame on the book. If an adolescent runs away after reading Huckleberry Finn or The Secret Life of Bees, that is a problem, since in neither case was the original meaning about running away. On the other hand, this is an adolescent who was quite likely thinking about running away, and it is difficult to blame a literary text for this. Yes, the original meaning was misunderstood, and this should serve as a cautionary example of the importance of understanding the original meaning, but these are extreme cases. If you read a book and interpret it to mean that you should be kind to animals, when the original meaning was meant to be an allegory for something else, there is really no harm being done. What you want to remember is that literature is literature and life is life, that the author controls the outcome in a literary text, but that you control the outcome in your life.
Often literature has a very different meaning in the context of the time period and local it was written in. A notable example is The Jungle, by Upton Sinclair. This book was written in protest of the poor treatment of the workers in the meat industry, but it was taken to be a protest of the quality of food being processed. Knowing that at the time worker's rights weren't nearly so well protected as well as realizing that the book had a massive impact on the meat processing industry, might prevent you from assuming that that is the current state of things.
In short, the importance and meaning of a book is always influenced by where it originates from while the context it is being read in is how you will primarily interpret it. Taking both the original context and the current context will allow you a greater understanding and make you a richer person for the knowledge.