Optimizing your website really is for one purpose, to attract customers to your website when they type in a keywords that is relative to your business in a search engine.
The best analogy for how Search Engine Optimization works is a summary on the back of a book. For this example imagine that you are an author and you have written a new book. You write a short summary of the book based on your angle and perception for the summary on the backcover. Now, imagine that a third party comes along and scans you book for the words that you have used the most often. this third party, or search engine, will then create its own summary based on those most frequently used terms. Not necessarily the content, but the most frequently used terms and how they are used.
When a potential customer visits a search engine and conducts a search, they are presented results based on these 'third party summaries' of your site. With this is mind, a well designed and optimized site will result in more visitors and more potential business as the search engines will know what content your site is about while serving these results up to people that complete a search.
Conversely, if you make use of words that do not relate to your business, you will not appear in search results as people are typing words into the search engine that are not used on your website, or not used often enough to register as being the primary keyword for your business.