what is the purpose of heating a porcelain crucible?

Answer 1

Crucibles are used to melt things at very high temperatures, ie. metal.

Answer 2

Heating (and then cooling) a porcelain crucible in a dessicator before using removes the water which has permeated the semiporus structure. Weighing the crucible before and after this procedure will show a weight loss. If your experiment involves heating the crucible to ash a product, your results would not be repeatabe because of the variable porosity (different amounts of water ab/ad sorbed) Even the use of platinum crucibles requires this procedure.