I heard that if a website can get a "do-follow" link, that will help the website to have a solid backlink. If there are many do-follow backlinks from the other website, it will give a good ranking at the search engine for a specific keyword. The website needs to have a keyword to compete with the other website, and if that website can gain more do-follow links, it would affect the rank on the search engine. If you stop the campaign, even if your link is still at the forum, your position in the search engine may be replaced by other websites because they still use the campaign to have more backlinks from the do-follow backlinks from the other websites or forum. But I don't know much about SEO.
You have to keep a balance between do follow and no follow backlinks, otherwise google will negatively think about it. Some people think that 50/50 is good while others think 30/70 is the most optimal (nofolow/dofollow).
Dofollow backlinks, to say shortly means that you give away your strengths to a website that you link. When search engine bots craw pages, dofollow backlinks give them a signal that makes them understand which website links which one. If you get dofollow backlink from bitcointalk's main page, once the bot checks it, it will hugely increase the rank of your website and it will appear far trustful for search engines than it was before because it got dofollow backlink from a very reputable website.