The drunken porter scene is meant to add comedic relief to the situation but he is not without a deeper significance. The drunken porter rambles on at length about drink being the great equivocator and how he could be seen as a porter at hell's gate letting in all of the damned souls. What is significant about his reference is the irony of the situation. The drunken porter speaks about letting in damned souls, but he is really letting in the protagonist of the play, MacDuff who is calling to wake the King. The porter was trying to be witty and clever, but hell actually is his master's castle now since Duncan is murdered beneath it's battlements.
It is also significant because he mentions two cases of damned souls and they all apply to MacBeth. The tailor and the equivocator were both traitors in life. Further, the tailor deals in clothing- and borrowed garbs and clothing is a lietmotif in the play.