It is important to establish the true weight of the crucible because it is a necessary reference point. Later, you will be weighing the crucible to determine how much of the products are present, so it's necessary to have the true tare weight of the crucible.
The crucible is heated to be sure it is free of any moisture or other volatiles that might otherwise give a misleading weight. If you start out with the crucible absolutely clean and dry, (after heating) then you know that any additional weight must have come from the sample.