Website statistics programs will tell you how many people found your website through search engines, which search engine they used, and what word or phrase they typed in the search box. These are critical pieces of information. People often search using some very unusual search terms that you might never think to include in your Keywords.
If you find that no one has come to your website using your top Keywords, you know that either you picked the wrong Keywords (words that no one is searching for) to optimize your webpage around or you did not do a good enough job in your Keyword placement.
If you find search terms that you have overlooked in the past, do some quick online research as described earlier to see how popular those terms are. If they are popular terms you can begin to develop new pages on your website to optimize for them, or include them in Long Tail Keyword phrases.