Since heading tags typically make text contained in them larger than normal text on the page, this is a visual cue to users that this text is important and could help them understand something about the type of content underneath the heading text. Multiple heading sizes used in order create a hierarchical structure for your content, making it easier for users to navigate through your document.
Let's say, for example, you're researching the keyword "how to start a blog" for an article you want to create. "Blog" can mean a blog post or the blog website itself, and what a searcher's intent is behind that keyword will influence the direction of your article. Does the searcher want to learn how to start an individual blog post? Or do they want to know how to actually launch a website domain for the purposes of blogging? If your content strategy is only targeting people interested in the latter, you'll need to make sure of the keyword's intent before committing to it.
You can confer some of your site's reputation to another site when your site links to it. Sometimes users can take advantage of this by adding links to their own site in your comment sections or message boards. Or sometimes you might mention a site in a negative way and don't want to confer any of your reputation upon it. For example, imagine that you're writing a blog post on the topic of comment spamming and you want to call out a site that recently comment spammed your blog. You want to warn others of the site, so you include the link to it in your content; however, you certainly don't want to give the site some of your reputation from your link. This would be a good time to use nofollow.
While SEOs can provide clients with valuable services, some unethical SEOs have given the industry a black eye by using overly aggressive marketing efforts and attempting to manipulate search engine results in unfair ways. Practices that violate our guidelines may result in a negative adjustment of your site's presence in Google, or even the removal of your site from our index.