Software was developed in the last 10 years to measure keyword density. Keyword density is the number of times a keyword appears on a page divided by the number of other words. It used to be claimed that Google favoured 4.7% etc. This is all water under the bridge as Google has improved its algorithm and we can now rank a website for a keyword without actually mentioning it on a home page.
However these (now free) word counting software are now useful in another way. What they can clearly map out is the use of language outside of a keyword cluster. This language gives our content writers the ammunition they need to create perfectly optimised content that Google should rank highly as this is what Google is expecting to see and rank highly.
Word comparison example
Here is an example of this website's most used words outside of common words like 'and' and 'the'. One ranks highly in Google and one does not. Should we be wanting to create a new SEO website then below would give us a guide on what content to create.