I completely agree with Tony sir that "Google brings the best, most relevant search results compared to any other search engine". Beside this, there are also some other factors that help Google to remain on top like:
Adopting new strategies and better technologies, very rapidly is the success mantra of Google. In the age of emerging technologies, one has to keep on providing new innovative products and services to its customers. Google is one of such a great organizations that have followed this techniques. For instance, Google's Semantic search engine is an upcoming technology that is going to change future internet surfing world, in all respects.
Google keeps on adding new apps to its products like Play new (Google Play) is the recent addition to its Gmail service and it has also added a Graphic calculator to its search engine, recently.
And regarding searching to any webpage for a search request, right now Google uses Literal search technique wherein, it returns web pages and documents based on matching for some or all keywords existing in input search text. This searching pattern also known as Navigational Search.
But in near future, with arrival of Semantic search engine technology, the searching in Google search engine will be made on the base of contents rather than keywords that will change existing SEO techniques, entirely. For more information on this topic, you can navigate to Impacts of Google's Semantic search on SEO.