I don't visit Fiverr a whole lot so I don't know about them specifically, but I have seen this sort of scheme before. Backlinks to your site increase your standing in internet searches. The idea being that the more sites that link to you, the more informative and interesting your site must be. In practice, it takes an enormous amount of backlinks to produce any sort of bump in search engine rankings. More importantly, "junk sites" don't generally count (and in the case of Google searches, will hurt).
There are companies that charge a fee to essentially spam your website all over the place. Mostly, they post the links on junk sites or use spambots to post links on blogs and forums. If you have a spamblock add-on for your blog, you might notice getting lots of "weird" comments that say something like "Your site is most informative will come back again hope you write more" followed by a link to something like shoes or whatnot. These are often from backlink packages trying to get the link posted on a popular site. They know that a lot of bloggers don't manually check comments or block comments, so they use the 'throw enough crap against the wall" approach to get the links posted.