Search engines can factor keyword density into their formula for determining the degree a web page may - or may not - be relevant to a specific search keyword or keyword phrase. Too many keyword occurrences can signal keyword stuffing and too few keyword occurrences can be an indicator of low significance within the context of the page content. Stuffing too many keywords into a page, or not using the right keywords, can negatively affect your site's SEO.
Term frequency - Inverse document frequency (TF-IDF) is a numerical statistic used to determine how important a word is in a document. Search engines can use tf-idf as a weighting factor for scoring and ranking webpage content relevance to a user's search query.
Run the On-Page Keyword Analysis on any web page URL before and after content changes, or take advantage of its ability to spy on and effectively check your competitor’s keyword usage and tf-idf.