Backlinks and in particular quality links are a very important part of any online marketing strategy.   As a site grows it needs a higher quantity of links to support itself, but the standard must remain high also.

Google uses what is known as a crawl budget to determine how many pages on a site it is prepared to spend time crawling and if the number of pages crawled is limited to only a percentage of you entire site, then straight away you have a problem.

Increasingly the quantity of quality links will in turn lead to an increase in your crawl budget, this means that very large sites need a lot of links to support themselves.  That makes sense otherwise it would be easy for a coder to quickly create a site with millions of pages and for them to rank equally quickly.   A site with a million pages needs thousands of high quality links to stand a change of all of its pages getting crawled.