Keywords are the first thing that you should know & learn about when start to optimize your blog SEO. If you fail on this very first step, the road ahead is very bumpy and most likely you will only waste your time and money. Imagine that your site have rich content, provides really helpful information for customers, but no one visit it, how can you earn money?
People now all seek their needs by search engine instead of visit a specific page, so optimize your content for high ranking is important now. If your keywords rank high, there will be higher probability customers will see and visit your site.
My advice when you write a blog post is that: Just pick up a relevant topic and write the post freely. After you finish, determine the keyword and find ways to put it in your post. Based on my experience, 4% in total words of your post are keyword is fine.
Hope this helps.
Best,
Will Smith.