How does the porter's scene increase tension and suspense in Macbeth?

In the scene just prior to the porter's, the audience witnesses the reaction of the Macbeths to Duncan's murder , and it is a very tense scene.  Macbeth is in a panic, thinking he'll never have clean hands again, and Lady Macbeth is chastising him for bringing the daggers from the room and calling him a coward for being unable to go back or pull himself together.  They hear the knocking at the door even as they are trying to wash their hands and calm themselves down.  So we are already feeling pretty tense even before we meet the porter.  

When the scene changes, we meet him.  He is very drunk, so when he hears the knocking, he pretends to be guarding the gates of hell, an ironic fiction because he doesn't know that all hell is about to break loose when he opens the door.  We, the audience, know more than he does (and more than Macduff and Lennox who have just arrived), and this generates dramatic irony (when the audience knows more than the character).  The purpose of dramatic irony is to create suspense, and so, as we watch them chit chat, we know what horror they are about to discover, and this builds tension for us.