When Update is upAlways, you can check this to know if really something visible changed since last update. If not, then you do not have to update the texture — no point, since it would look exactly like the current one.
Scene classes (TCastleSceneCore, TCastleScene, TGLRenderer) take care to set this field. After each actual update of the texture, it's set to False. Each time something visible affecting the look of this texture possibly changed, it's set to True.