SEO is similar like internet marketing. Well, the important is to have a decent proportion of good backlinks.
Because a backlink on a strong PR page, even unrelated will bring bots and visitors to your site for sure. If for example you could have a link on phpBB home page (PR 9), there is now way it could hurt your Ranking, what we would not want would be backlinks only coming from unrelated websites or worst, only coming from a single domain being unrelated. That's why backlinking is a long and never ending run, we should consider every occasion we have to make more and set priority among them to always work on the best one we can actually obtain.