Short Desc: one of the main pillars of SEO is ensuring that keywords are used on a website.
Keyword Research is vital for a successful SEO strategy, but knowing the keywords and tracking them in a website ranking tool accomplishes nothing unless you use these keywords in key areas on your website. Google 'reads' a site to understand it - the frequency of words is one indicator to what a site is all about. Google also applies 'weight' to words used in high profile areas:
<H1> <H2> <H2>
<img src="/image.png" alt="alternate text here" />
<meta name="description" content="120 character description here" />
It is important to 'drip' your keywords throughout a page, from the title, down to the H1, in a couple H2 headings and naturally throughout the body copy.
But it is equally important to not have useless headings. We frequently see headings like:
This text may be important for the reader, so by all means, keep it big in size - but it should never be declared a heading as it instructs google that this is important text - and from a search context it is NOT.