1. The natural language people use in their testimonial or review of your product or service helps your ranking in search engine results. If you are ranked high in search results more people will find you.
2. Product or service reviews and testimonials often help to describe your product or service better than you can.
3. When your customers describe why they selected your product over the competitions it sounds a lot better than when you say the same thing. Let your customers be your champion.
4. The honest feedback from your customers can help you refine your product or service strengths and help you to identify and fix weaknesses.