Halakhah su II Re 4:2

Do not remove the tablecloth or bread until after Birkas Hamazon. Bread should be on the table at the time of Birkas Hamazon to demonstrate the abundance of goodness from Hashem, blessed be His Name—that He gives us enough to eat and to spare, as Elisha [the prophet] said to his servant:²Kings II Chap. 4: 43. "For so said Hashem, eat and leave over." Another reason for this custom is that Hashem's blessing has no effect on a void. It is only effective on some existing substance, as Elisha said to the wife of Ovadiah:³Kings II 4: 2. "What do you have in the house?" [which I can bless.]