In this video Matt Cutts reveals that Google does not use keyword for ranking “even the least little bit”. It looks like so much spamming had been going on that keywords were effectively rendered useless for ranking sites and pages.
There are two other use cases that spring to mind. Accessibility and composition.
In the UK, the Royal National Institute for the Blind recommends the use of metadata, including keywords, to aid navigation.
“Page information metadata must include a page TITLE, keywords, author and description. This should be as accurate as possible to ensure that users get all the information they need to navigate a site.”
Keywords can also provide a useful focus when composing text for a page. It’s a way of noting the few precise concepts you want to cover in a manner that will survive edits and revisions you make in the text.