SEO perspective with No-follow tag

Originally, the nofollow attribute appeared in the page-level meta tag, and instructed search engines not to follow any outgoing links on the page.Before nofollow was used on individual links, preventing robots from following individual links on a page required a great deal of effort. That's why the nofollow attribute value of the rel attribute was created. This gives webmasters more granular control: instead of telling search engines and bots not to follow any links on the page, it lets you easily instruct robots not to crawl a specific link. Google does not transfer PageRank or anchor text across Nofollow tag links. Essentially, using nofollow causes us to drop the target
links from our overall graph of the web. However, the target pages may still appear in our index if other sites link to them without using nofollow, or if the URLs are submitted to Google in a Sitemap. Also, it's important to note that other search engines may handle nofollow in slightly different ways.Untrusted content: If you can't or don't want to vouch for the content of pages you link to from your site — for example, untrusted user comments or guestbook entries — you should nofollow those links. This can discourage spammers from targeting your site, and will help keep your site from inadvertently passing PageRank to bad neighborhoods on the web. In particular, comment spammers may decide not to target a specific content management system or blog service if they can see that untrusted links in that service are
nofollowed. If you want to recognize and reward trustworthy contributors, you could decide to automatically or manually remove the nofollow attribute on links posted by members or users who have consistently made high-quality contributions over time.

Paid links: A site's ranking in Google search results is partly based on analysis of those sites that link to it. In order to prevent paid links from influencing search results and  negatively impacting users, we urge webmasters use nofollow on such links. Search engine guidelines require machine-readable disclosure of paid links in the same way that consumers online and offline appreciate disclosure of paid relationships. More information on Google's stance on paid links.Crawl prioritization: Search engine robots can't sign in or register as a member on your forum, so there's no reason to invite Googlebot to follow "register here" or "sign in" links.
Using nofollow on these links enables Googlebot to crawl other pages you'd prefer to see in Google's index. However, a solid information architecture — intuitive navigation, user- and search-engine-friendly URLs, and so on — is likely to be a far more productive use of resources than focusing on crawl prioritization via nofollowed links.Internal Linking: Internal linking to pages sometimes doesn’t need to be a followed link. For example, if you are linking to a contact page or maybe a testimonial page that might not need to
have link power passed over to it, however, if you are linking to an important service or product page than you will want to leave the do follow status on it.