Don't be fooled by talk about security or privacy. This is simple Nation State realpolitik. TikTok is a propaganda threat controlled by a non-friendly state to the US. Any other way to look at this is naive.
Is there any evidence it's been used for propaganda? I don't use TikTok a lot, but it seems very non-political (maybe it's my filter bubble). The real cesspool of hatred and madness is Facebook - but of course Congress doesn't care too much about that. As far as I remember from the previous elections the Russian bots were operating on US based platforms
On the opposite side you would fill it with messages about the virtues of hard work, stories of success and happiness, etc.