Why is the Introduction important?

Your introduction shows if you are going to provide the kind of response which is either:

• what the examiner expected when setting the question, or
  
• not what the examiner expected - but if you have justified or supported your position, the examiner will be more likely to give credit for your own interpretation.

You have to establish the relevance of your material in your Introduction, and then stick to that material throughout the essay.  You are also expected to provide the reader with a "map" of where your essay is going, and of what it will cover.   We call this "advance signposting", and deal with this more thoroughly here