Basic purpose of search engine optimization is to bring audience to your blog or website, get their attention in a natural way, provide them your services or products and make money out of them. If your blog is such kind of blog and your ultimate goal is to make money from your blog or generate a loyal readership to convey your message, then you should must do search engine optimization of your blog or website to get audience on your blog from search engines.
But, if your blog is sort of personal blog, where you just share your thoughts and experiences and you don't mind if anyone reads it or not, then you may don't spend your time on doing search engine optimization for that personal blog.
You might think why one won't like free traffic on blog, then just for information, there are people over the web who write blog for themselves only, just to archive their life and experiences. This is why popular blogging platforms like WordPress and Blogger have built in features where you can adjust blog settings and stop search engines to index your blog posts.
So, since most of the web has become kind of commercial channel and for-profit platform, hence most of blogs and websites must do SEO in order to stay ahead of their competitors in terms of SEO. Additionally, a particular type of blogs that do not need someone's attention like personal blogs etc.
You can read step by step guide to search engine optimization that is designed in such a way that beginners as well as experts can take benefit of it.