SEO can have a big impact on your website. However, there are never guarantees that more visitors means more customers. There are lots of things at play, the landing page, how you drive customers to your product, whether it is online, or at your business location. Generally speaking, more visitors to your website often correlates with more customers, and in my experience, it is closely related. However, there are cases when someone is selling a product or service, that could be above market value, poor name recognition, etc. and they are not seeing the same success.