Video tagging is useful for several reasons:
Because of these benefits, YouTube users (read: uploaders) were given the option to tag their videos with relevant keywords, and viewers could see these tags on every video. This feature, however, has been discontinued today.
According to a recent post on Google Plus, YouTube had to put an end to video tagging because “viewers weren’t using them, and some abused them.” Though video uploaders can still add tags, they will not be visible to the viewers.
You may have noticed – we made tags private, so you can’t see them on the watch page anymore. Viewers weren’t using them, and some abused them. Please keep providing tags when you upload, though – they’re important tools to help promote and connect your videos!
Although tags were always buried under the video description, they were taken into account for ranking purposes. Many folks who wanted to bring traffic to their videos, tossed in tonnes of irrelevant tags on every video, thus deceiving YouTube and its viewers.
Probably disabling video tags owill trim down the spamming activities on YouTube.
Via YouTube Creators