But ByteDance, TikTok's owner, operates it's own analogous app in China, Douyin
Reasons for having two apps are rife with speculation. One is that censorship in Douyin is more prevalent than on the export version (that one is pretty obvious). There's also speculation that the export version of Douyin has an algorithm tuned to be more addictive.
But let's be clear, by ByteDance's own statements the two apps have shared management and technology.