Meta Keywords.. Why?

I've just read through 20-30 posts from 2009 to present, looking for an indication of whether meta_keywords have now, ever will have, or ever DID have, any relevance at all. It seems they don't

I'm left deeply confused. Why is there a field for them in every SEO Plugin out there? What are they used for other than pay-per-click?

Any SEOs out there shed some light on this? Many thanks.