In 2005 Google announced the
rel=nofollow link attribute. They said that using it would make Google ignore the link. Googlebot would not pass PageRank across the link. For search engine ranking purposes, it was like the link wasn't even in the page. The purpose of nofollow would be to mark links that could be spammy such as links in user generated content.
That lead many webmasters to see nofollow as a tool to control link juice on internal links within their own site. The thinking was that adding nofollow to an internal link would cause Google to pass link juice only to the important internal links on the page. It turns out that thinking was misguided.