In the past, it used to be handy to put a bunch of keywords into the keywords meta tag in your html code (for example, ). Search engines then used these keywords to help understand what YOU considered most significant on your web page.
Spammers used to stuff this tag full of completely inappropriate keywords. This, coupled with the fact that search engines are now so much better at understanding what a page is about just from the page content itself, has caused Google at least (and possibly others) to stop considering these keywords.
So why not spend all of the time you save by sharpening up the content on your pages?