It probably started with the guy adding something like "Edited using XXXX Editor tool" to make himself some publicity. Seeing that it worked he started selling those backlinks a fortune.
Circa 1999 I was running a webdesing studio. We added that link to all the websites we designed, then the next logical step was to make it link to a page with our entire portfolio which in turn linked to our website. That boosted the SEO of all our customers, and in turn boosted ours exponentially.