Think about it. You're looking for something on Amazon. The product looks great in the photos, and the product description is reassuring. But wait a moment... How many times have you bought something online and had it delivered, only to be disappointed with the quality?
So, nowadays before buying, you always look at the review score, and if you're spending more than an impulse purchase amount, you scroll through a few reviews and see what past customers had to say about the product before buying.
Well, your potential future customers are just the same. By not having credible, 3rd-party customer reviews somewhere on your site you'll never know how many customers you could have had. Instead, they're heading to one of your competitors that does have reviews on their site and allows past customers to adequately reassure new customers that they'll not regret parting with their cash.