Search engines pay attention to links between sites, these are often called link exchanges, and they help a site in the search engine.
Why do they help? Because sites that have strategic alliances with other sites are generally good solid sites. On the other hand link exchange farms are not considered valuable. Many link exchanges are attempts to mimic the link structure of a strategic alliance – from this point of view these links are spam; that is the point of view held by the creators of algos who try to discern the link exchanges. Spam is a hard word, actually many directories which are “useful” resources exchange links without much thought to the site. That is the question that needs to be asked is the exchange of links useful for visitors.
What is a good strategic alliances; one that actually exchanges traffic that converts into customers for both sites.