In recent years resource links have grown in importance because most major search engines have made it plain that -- in Google's words -- "quantity, quality, &relevance of links count towards your rating."
The engines' insistence on resource links being relevant &beneficial developed because many of methods described elsewhere in this article -- free-for-all linking, link doping, incestuous linking, overlinking, multi-way linking -- &similar schemes were employed solely to "spam" search-engines, i.e. to "fool" engines' algorithms into awarding sites employing se unethical devices undeservedly high page ranks and/or return positions.
Despite cautioning site developers (again quoting from Google) to avoid "'free-for-all' links, link popularity schemes, or submitting your site to thousands of search engines (because) se are typically useless exercises that don't affect your ranking in results of major search engines -- at least, not in a way you would likely consider to be positive," most major engines have deployed technology designed to "red flag" &potentially penalize sites employing such practices.
