Where these tags come from
YouTube stopped displaying tags publicly on the watch page itself back in 2012, but it still writes every
video's tags into the page's static HTML - in a <meta name="keywords"> tag and as individual
og:video:tag lines - so that search engines can read them. This fetches that public HTML and pulls
the tags out directly; nothing hidden or private is accessed.
Why this is useful specifically for competitor research
Use this for competitor research - seeing the tags successful videos in your niche use can inform your own tagging strategy - keeping in mind that tags are a minor ranking factor compared to titles, descriptions, and especially audience retention. A tag list alone won't explain why a video performed well, but it can surface topic variations and phrasing worth considering for your own videos.
Why this is a different tool from this site's YouTube tag generator
This tool reads the tags an existing, already-published video actually used, while the tag generator builds a fresh list of suggested tags for a video you're about to upload - one looks backward at what's already out there, the other looks forward at what to add, which is why they're built as two separate, purpose-specific tools rather than one combined one.
Why finding no tags for a video doesn't necessarily mean anything unusual
Either the uploader genuinely never set any tags, or YouTube simply isn't including them in that particular page's static HTML at the moment it was checked - both happen regularly and neither points to anything wrong with the tool or with how the video itself was set up.