This is one of the most common SEO mistakes. Even today it’s quite stunning that many web developers and website owners make no effort at all to perform research into the keywords for which they should rank highly.
This completely dilutes the thematic relevance of the page. Of course, website owners then wonder why they don’t rank well for any of the key phrases they’ve selected.
If you target one or two “on theme” key phrases for each page, the page will be highly relevant for those words and you are more likely to rank higher in search results.