useWatch is not only for useData: an asset announces itself when it lands, so the placeholder is swapped for the picture by a callback that runs exactly twice instead of a check made sixty times a second. Reload the page to see the loading frame, which is held back on purpose here because the file is already in memory.
What cannot be watched is anything the game moves by hand, a position above all: nothing announces those, so useWatch refuses them when the scene starts rather than going quiet.