This is very important for SEO. Keep in mind that the search engines are just large computer programs digesting your site and trying to figure out what it is about. One of the simplest things they do is to count up all the words and look for repeats. They then calculate percentages, or densities, of specific 1-word, 2-word and 3-word phrases that are found in your text. By looking at the most popular keyword phrases, their programs understand the important themes of your page. If you observe your own writing on a specific subject, you will see the patterns as well.
I don’t recommend that you write solely with keyword density in mind as it will result in lower quality content. However, I also don’t recommend that you completely ignore keyword density in your content creation. My preferred approach is to write content straight-up for the first draft. Then, as you edit for grammar, consistency, and clarity, also edit for density. Run your content through a density checker and see what phrases are used the most. Make adjustments accordingly so that your top themes / keyword phrases are showing up between 2-4% of the time. But don’t do this to the extent that anything reads as unnatural. You need to always keep your audience in mind.