In his book, Optimal Investing (Affiliate Link), Scott Frush writes this about the importance of having an Investment Policy Statement:
Much like a blueprint for building a house, an Investment Policy Statement serves as the blueprint for building your optimal portfolio. This policy is crucial to the long-term achievement of your specific financial goals. First and foremost, an Investment Policy Statement helps you learn more about what your needs and priorities are, how to best address them, and the risks involved with investing. Secondly, this policy allows you and your portfolio manager (if you elect to employ one) to gain a better understanding of your objectives and constraints and how to best manage your portfolio to accomplish your specific financial goals.
A written Investment Policy Statement will not alone guarantee success in protecting and growing your optimal portfolio. Rather, it will shelter your portfolio from ad hoc revisions, made by either you or your portfolio manager, from a sound long-term asset allocation policy.
Basically, an Investment Policy Statement should explain why you’re investing (your goals) and what you are investing in. Why is this important? Because human nature tends to take over when times get tough and might cause you to make changes to your investment plan based on emotion rather than sound logic. Being able to pull out and read through your Investment Policy Statement (IPS) will give you comfort and just might keep you from making a serious mistake.