What basically does is compare your content with the content of the top say 10 websites (you have to exclude big websites that are not ranking because of the proper SEO like wikipedia, brand sites etc). If your keyword research Kung Fu is good and your content writers decent the suggestions are pretty much useless. Also tables and elements with products/services add up to the keywords and if you are not careful can actually harm your site as Surfer suggest you way higher keyword density, which in reality doesn't exist in the content. All in all if your keyword research is good and the writer follow instructions, the Surfer wont help you much.