Main principle of NoFollow attribute is to teach the search engines that a specific link for which this attribute has been enforced has no added advantage of search engine rankings. This NoFollow attribute is designed especially to stop the 'cold war' between search engines and spammers, who do everything to get high page ranks in search engines, even if their pages are of inferior quality.
According to Google, each paid link needs to have NoFollow attribute, since an individual or business concern need not reach the top place of search engine by purchasing many links. Other links, which have this no follow attribute, are those links in comments of a blog system, guestbook and links of external sites that are irrelevant to the particular web content (for instance, link pages and link directories).
Is it Necessary:- Public forums are susceptible to link spamming, while other discussion forums have their personal way to deal with such problems. Mostly, blogs also known as personal commentaries are unable to avoid spam, since they are mostly one-man efforts. Hence, spammers take undue advantage of this by sending junk emails.
Spamming with superfluous comments by spammers, whose only purpose is to have high page rankings through posting backlinks in web sites, have now become a real hazard for many web site owners. Thus, bloggers need to face the burden of this blitz. Often, blanket spamming daunts those who are able to offer genuine comments.
So, what is the solution to all these problems? According to many professional SEO consultants and SEO service providers, NoFollow attribute in your web site can help you avoid those unwanted link spamming. As tracking and indexing tools such as Google's NoFollow attribute tend to relate backlinks as manifestations of support, they evade those problems of link spamming in your web site.
Benefits:- With NoFollow attribute, many well oriented web site owners and bloggers enjoy the pleasures of blogosphere with no link spamming in their blogs and sites. In addition, they have also witnessed high Page Rank without any link spamming. Hence, using NoFollow attribute, web site owners and bloggers can easily attain a high PR in major search engines without the fear of unwanted link spamming.