Untagging has become a more important feature in recent days because some users have noticed an influx of spam videos that they are tagged in. These videos might be coming from bots, or from user accounts that are unaffiliated with the user being tagged in them. Either way, many users understandably don't want themselves tagged in these videos, no matter what the content is.
Untagging may only be a small scale solution, but it's all that some people need, at least for now. Hopefully TikTok finds a way to get spam under control so that users who are tagged in these videos still enjoy using the platform, and don't find themselves inundated with spam every time they log on.