Why does Falstaff constantly lie and boast about himself even though honor is useless to him?

Falstaff's words about honor: “Can honor set-to a leg? No. Or an arm? No. Or take away the grief of a wound? No. . . . What is honor? A word”

He also lies about his encounter with Poins and Hal when they were in disguise (Act 2, Scene 4). At first he lies that he fought very bravely and gradually increases the number of people he killed, but after Poins and Hal reveal that it was them and they saw it all happen, he says that he just acted to be a coward and he would never harm the next King.