The purpose of Search Engine Optimisation is to improve your web pages in a number of ways in order to optimise their chances of being picked up by a search engine for a relevant user search, and then appear as highly as possible within the ranked search results. One of the ways to do so is to optimise a page for a keyword or key phrase that you want to target, by including that word prominently in as high a volume as possible of text upon the page.
Search engines love text. After all, when a user punches a search request into a search engine, they do it using text, and it is that text which the search engine instantly canvasses the internet for. If your site features the appropriate keyword or phrase, in high volume without sacrificing the quality and readability of the text, you will improve your chances of appearing in a high search results position.
If you’re going to optimise your site in this way, however, heed the following warning: don’t attempt to optimise a single page for a variety of keywords or phrases at the same time. This greedy guts approach to SEO is likely to hinder your site’s chances of a high results ranking, rather than improve them.
A search engine doesn’t have a brain, just a set of functions; a set of algorithms. When a word is searched for, the search engine rounds up the results for that word. If a particular page features that word in high volume, within original coherent text, this site will appear in a high position. This is a largely mathematical phenomenon designed to reflect the importance of that word or phrase within the page. If the page is trying to rank for too wide a variety of words and phrases, no definitive subject can be drawn from the page, mathematically or thematically.
This is why sites with good optimisation focus different pages on different keywords to ensure that particular page ranks well for that particular keyword or phrase.