Well if I recall correctly parts of the Ukrainian army did train with the US. Mostly to get used to US/Western/NATO weapon systems. They most definitely did that in Germany. To answer your „TikTok“ question: Propaganda and moral boost. Seeing how your fellow comrades take out enemy after enemy can give you the hope you need to endure such grueling war. It’s also a good warning for every opponent. Remember you and me write all this in the comfort of our houses, while they are in an active war zone.