Great sites have some unique components, what makes them outstanding from overall crowd. Site graphic may consist many elements, including typography, layout, background graphic, icon sets and many other details, which distinguish site from hundreds of similar others.
Historycally, there was more original approach to design all sites from scratch. It was time, where sites were carefully handcrafted and field was rather art than mass production. Certainly the prices for this kind of approach was much higer and time for creating sites a lot longer.
Since understanding what are websites functions and what kind of communication goals they should fulfill are more pragmatic nowadays, website designers offer to clients also cheaper and faster solutions. There is not much difference in overall user experience, but rather in production process itself. Some functionalities are quite similar from site to site so there is no need for reprogramming them from start each time. Similarly page layouts are very typical for certain amount of information, so users expect articles and links to be presented in similar manner for better usability.
Original approach has still its nische nowadays, for example in arts and advertising fields. For practical delivery of information, users expect some standards, which doesn't require re-learning navigational principles each time they enter to new website.