Why should anyone care? Well, because Google and other search engines do. Everything else being equal, the more frequently you appropriate use a keyword the more relevant your document is likely to be when someone runs a search using that keyword. So the more you use that keyword, the higher in Google's search rankings that word appears.

The same is true for many of the better applicant tracking systems and job boards as they have built-in search engines. If your resume or job posting ad includes the keywords that your targeted user is most likely to enter when searching for your resume or posting then your resume or posting will come up higher in the search results and therefore be more likely to be read. Of course, if your document doesn't contain the keywords at all then it is unlikely to come up at all. So rather than posting a document that includes a keyword phrase such as "registered nurse" but not "RN," post a document that includes both. And preferably include a few references in the document to those keywords if those are the keywords most likely to be used by someone searching for the information that you've posted.