because i have some large textures that i want to unload in a certain condition, and don't want them to be loaded accidentally.
i can write code like this:
if (Texture.isLoaded(frameBuffer)) TextureManager.unloadTexture(frameBuffer, texture);
but , i have other way : i can hide the object3d that has the texture, then the texture won't be loaded accidentally. so it's OK without knowing the flag.