Question: There is a study that users do not go past top 3-5 pages in a Google search, on which predominantly everybody would agree. If you would get your best results - it means to be within the top 5 page at least. Merely users abandon to search or change the query if the results do not get laid in the first few pages. In that case, why do we have MORE search results, pages 10, 11, 12, ... and beyond? What meaningful impact does it serve when users rarely do get to that page? Is there any useful content lying far deep?

Answer: 
It's a way to improve search result quality by letting users explore information found in long tail of the result.

More importantly, having long tail mitigates a dangerous feedback loop in which the first few pages results get disproportionately higher ranking.
Users often do not find a good result in the first page - this depends on how specific of a result they are looking for and how many false matches fit their query.

I personally often go through a few pages of search results before refining the search query to try and improve the quality of the results.