A good search engine does not attempt to return the pages that best match the input query. A good search engine tries to answer the underlying question. If you become aware of this you’ll understand why Google, use a complex algorithm to determine what results they should return.
The factors in the algorithm consist of “hard factors” as the number of backlinks to a page and perhaps some social recommendations through likes and +1’s. These are usually external influences. You also have the factors on the page itself. For this the way a page is build and various page elements play a role in the algorithm. But only by analyzing the on-site and off-site factors is it possible for Google to determine which pages will answer is the question behind the query. For this Google will have to analyze the text on a page.
Source: Moz
