Backlinking is an important tool in building up credibility for a given website. The basic rule of thumb is, the more backlinks a site has, the more authority it has. Of course, by todays standards the SEO algorithms are much more complicated then looking at sheer volume. The credibility of the website providing the backlink is also used in determining the strength of the backlink.
SEO fundamentals teach that backlinks should be gained in a natural and organic fashion. Backlinks that are purchased, created via bots, or otherwise obtained via disingenuous routines often hurt the credibility of the linked website.