Your introduction and even your first sentence are the most important part of your personal statement. This is where you can grab the attention of the admissions representative. Make your first sentence unique and compelling. It is recommended that you state in a creative way WHY you want to undertake this field of study in your first sentence. The rest of your introduction should provide a brief explanation that supports this first statement. Divide the word limit by the three parts of your personal statement by allocating your introduction around 30%.
When you allow a fresh set of eyes on your admission essay it is almost as if your personal statement is getting practice. After all, your statement will ultimately end up being read by one or more people while your application is being reviewed so it makes sense that a well read personal statement should do better in its final stages. Why have the reviewers catch the mistakes that someone else can catch beforehand? It is far better to catch a small mistake that can cost you admission, before it actually has the chance to cost you.