Offering a strong guarantee with your product shows that you believe in your product, and gives users the confidence to give it a try, knowing that their purchase is guaranteed in case something goes wrong or if they're unhappy with their purchase.
100% money back guarantees can be a great tool, but use these sparingly. Chances are that most people won't take you up on it unless your product is terrible, but the upside is much higher. 5% of users might abuse this, but 95% will see value from it, and in turn you'll see your conversions improve.