You Will Narrow Down Your Options
By knowing well what you want and what you don't, you'll find that your list of options has been greatly narrowed. This will help you to focus and refine your job hunting tactics.
You Will Have A Better Knowledge of Yourself
You will have better confidence, in coming to terms with yourself; and it will show. Interviews will run much more smoothly, as you will effectively able to communicate your strengths and weaknesses.
You Will Avoid Unpleasant Surprises
This ties in with the first statement. You don't want to get hired for a position in a company that absolutely does not fit you. You can avoid that trap easily, if you just remain honest with yourself, and make your choices accordingly.