When would this be useful? If your site has a blog with public commenting turned on, links within those comments could pass your reputation to pages that you may not be comfortable vouching for. Blog comment areas on pages are highly susceptible to comment spam. Nofollowing these user-added links ensures that you're not giving your page's hard-earned reputation to a spammy site.
What is Search Engine Optimization (also known as SEO)? A broad definition is that search engine optimization is the art and science of making web pages attractive to search engines. More narrowly, SEO seeks to tweak particular factors known to affect search engine standing to make certain pages more attractive to search engines than other web pages that are vying for the same keywords or keyword phrases.
Great post. I know most of the stuff experienced people read and think “I know that already”… but actually lots of things we tend to forget even though we know them. So its always good to read those. What I liked most was the broken link solution. Not only to create a substitute for the broken link but actually going beyond that. I know some people do this as SEO technique but its actually also useful for the internet as you repair those broken links that others find somewhere else.