Consider the age of your business. If you just opened your business and launched your website, it’s going to take time to develop your SEO and begin to appear organically in the search. While that doesn’t mean you shouldn’t put together an SEO strategy, it does mean that you could benefit from an SEM strategy until you build your SEO. SEM is an effective way to drive traffic while building organic SEO.
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.
As an Internet marketing strategy, SEO considers how search engines work, the computer-programmed algorithms that dictate search engine behavior, what people search for, the actual search terms or keywords typed into search engines, and which search engines are preferred by their targeted audience. SEO is performed because a website will receive more visitors from a search engine the higher the website ranks in the search engine results page (SERP). These visitors can then be converted into customers.