Why empty crucible should be heated?

You have to fire a crucible to a consistent weight to ensure that you have an accurate measurement. This is often done when high precision is needed, and you have to make sure that you have a completely clean, dry crucible so you know that every bit of the weight comes from the sample. you wouldn't get an accurate result if you had a little bit of something in the crucible, and then when you burned off the paper or something, it would remove some weight so that you'll think you have less than you actually do