6.Obviously in all comment sections, if we don’t know with whom we are dealing with. And in the wide world of the internet we really never know for sure. If we set comment section on nofollow, we will not pass authority of our pages to unknown addresses.
7. If we sell space in our pages, we should put nofollow attribute on links too. If we don’t do that, we are actually selling our authority and this is big no no according to Google webmaster’s guidelines.
Google has two rankings of the sites on the web, one comes from so called organic searches (most of humans are made from organic compounds) and it heavily depends on authorities of backlinks. Other ranking is paid through Google AdSense program and all links on this rankings are nofollow.
8. It is logical, although many web marketeers don’t know that, to put nofollow into all affiliate links. With affiliate links we are trying to sell product X, available on page Y, through our page Z. But if we set a link from Z to Y, we are actually voting Y up and in search engine’s rankings Y will be higher than our Z. This means potential buyer would more likely click on Y than our link and maybe buy a product, but we would not earn any commission.
9. It can be very good idea to use nofollow tag on unrelated content. You can check our first site in the next picture. As you can see we use a lot of free images to make our site more attractive. But images are not royalty free and this means we have to give credit to sites they came from in form of backlinks.
Unfortunately spiders, crawling through our site, saw a lot of links to authoritative sites dealing with photography and got an impression our site is dealing with photography too. It took us couple of months to find a problem and told search engines what is our site really dealing with.