The keywords meta tag is useless
Why the Keywords Meta tag is totally and utterly useless.
There was a time where webmasters where to set the keywords of their websites, using a certain keywords tag. This was however abused by a lot of webmasters, to spam the SERPs, so search engines started to ignore it entirely, as an example to follow.
The keywords meta tag has little use today, and is practically worthless. Today search engines use other much more precise methods, to determine the relevancy of websites.
When to use it?
There is practically no thinkable situation, where a webmaster would still include the keywords meta tag. That is because of before mentioned reasons. Yes some sites likely still use it, but they are likely also to clogged with junk, for any user to be interested in using them.
Instead you should focus on other aspects of SEO, mentioned in Brugbarts SEO Tutorial.
Abused for spam
Search engines has become very sophisticated in analyzing content of web pages, which means that it has become very difficult, to get a decent ranking keywords unrelated to your pages.
There was a time where some webmasters, would simply fill the keywords tag, with a bunch of unrelated keywords, in a pony attempt to drive more traffic to their sites.
Other abusable tags
There are arguably other tags which can be abused, but not nearly as easily without getting spotted, and abusing those may servilely harm the placement of a website. Then you might ask, well why not just compare the content of the keywords tag with the content of the page? Well its more efficient to simply analyze the content alone, since it obviously needs to be checked anyway.