For Search Engines, backlinks help to determine the page’s importance and value (i.e. authority). Historically, the quantity of backlinks was an indicator of a page’s popularity.  Today, due to the way backlinks are evaluated based on different industry-related ranking factors, it is less quantity focused and more about the quality of sites from which the links are coming.

We must be careful with our reciprocal links. There is a Google patent in the works that will deal with not only the popularity of the sites being linked to, but also how trustworthy a site is that you link to from your own website. This will mean that you could get into trouble with the search engine just for linking to a bad apple. We could begin preparing for this future change in the search engine algorithm by being choosier with which we exchange links right now. By choosing only relevant sites to link with, and sites that don't have tons of outbound links on a page, or sites that don't practice black-hat SEO techniques, we will have a better chance that our reciprocal links won't be discounted.
While ordinary users were not that interested in pages' scores, SEOs of a different caliber felt that this was a great opportunity to make a difference for their customers. This obsession of SEOs with PageRank made everyone feel that this ranking signal is more or less the only important one. In spite of the fact that pages with a lower PR score can beat those with a higher score! What did we receive then, as a result?

Page Rank