Those who make websites, especially those who do SEO, know that the quality of a website's friendly links is very important, and the quality of the homepage's links is even more critical.
Simply put, if Site A has a link to Site B, and Site B has a high authority (measured by PR), then it is quite easy for such a link to increase the authority of Site A as evaluated by Google PageRank (PR). As a result, the relevant content of Site A will be more likely to rank higher. Many sites work hard to improve their PR based on this principle, and some even spend money to buy links in order to boost their PR.