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

As in other tragedies, such as Hamlet, there are scenes that seem rather non-sequitur, perhaps inserted for comic relief or for dramatic effect. Such is the porter scene of Act II.

While there is great tension in the previous two scenes of Act II as Macbeth kills King Duncan in the first scene and then is overwrought in the second as he imagines that he hears someone crying out about sleep having been "murdered."...

As in other tragedies, such as Hamlet, there are scenes that seem rather non-sequitur, perhaps inserted for comic relief or for dramatic effect. Such is the porter scene of Act II.

While there is great tension in the previous two scenes of Act II as Macbeth kills King Duncan in the first scene and then is overwrought in the second as he imagines that he hears someone crying out about sleep having been "murdered." Scene 3 continues this tension with the repeated knocking, while adding some comic relief in the character of the porter.
The knocking at the door is, perhaps then, the objective correlative of the beating of the Macbeths' hearts while they wash the blood from their hands while they speak of other things that they have heard, such as a voice crying, "Sleep no more! Macbeth doth murder sleep!" (2.2.34-35)

In Scene 3, then, the tension is objectified with the loud knocking; further, the horror of the murder brings suspense and gains intensity with the coarse vulgarity and comic relief of the Porter. In addition, the incongruity of the foolish porter and the "devil-porter" with Inverness as an Inferno, as well as the allusions to "the equivocator" (Father Henry Garnet, a conspirator in the Gunpowder Plot), throw light upon the moral depravity of the act of regicide just committed.