So it's only used within Calibre (the Calibre library manager).
For example, putting them in and then creating the epub and then uploading that to Amazon will be meaningless for any search engine to find it. KDP's tags are used for that.
It was created for readers, but works for published works as well. Again, the 'tags' field consists of SEO information (Search Algorithm Tags), so list any related search terms you think might be useful. But I put a LOT of effort into my SEO terms, checking to see which produce the biggest effect without becoming meaningless.
However, while the published book will list the tags, you'll want to register the SEO tags WHEN you publish (say on Amazon, lulu or smashwords), so including them in the epub is really a duplication of effort.