The most practical use not mentioned here is probably to import existing trained models/weights. I can see it being useful for anything that you want to run in real-time (e.g., webcams apps like https://github.com/ModelDepot/tfjs-yolo-tiny) and can't pay a round-trip cost to server.

https://js.tensorflow.org/tutorials/import-keras.html

Training a model in the browser is the least practical use for tensorflow.js IMO (unless maybe you want to hijack people's browsers to help with training or something).