Normally people use search engines to find what ever it is they are searching for. Thus, optimizing a site to be placed at a high rank is crucial.
So, John Bob wants to find a private computer tutor... he goes to Yahoo and types "private computer tutor". On the first page he finds "webshahar.com"; then types computer wizard and finds "webshahar.com" again.
Wouldn't it be nice if this were to happen to you too?
As this is just an example of what search engine optimization can do.
Small list of search engines