In Act 2 Scene 2 of Macbeth, What is the dramatic importance of the Porter scene and 'the knocking on the gate'?

The porter scene or the discovery scene (Act III Scene III) in
Macbeth has attracted many critical commentary and conjecture. It comprises of two
climaxes – the comical porter’s apparently irrelevant and tipsy comments and the
discovery of the treacherous murder of Macbeth’s guest, King Duncan.
The
Satirical porter scene written in earthly prose is intended a comic relief in the grim
tragic atmosphere. The sordid, tense and serious atmosphere of conspiracy and murder is
slightly eased by the humourous speeches and incidents of the porter. It is woven into
the drama in such a way that they have widened and enriched, rather than weakened, the
tragic significance. Alike the gravediggers in Hamlet, the speeches of the Fool in King
Lear, the Porter’s nonsense verbatim aims to relieve the tension and hightens the tragic
element by contrast.
The porter is  in drunken state and imagines in the Hell
Gate. The castle of Macbeth is alike hell and villainy of Macbeth has invested it to its
utmost notoriety. Thus the irony in Porter’s speech can well be read. The porter next
fancies that three men, a farmer, a Jesuit equivocator and an English tailor knock for
admission. Commenting on the farmer, the porter says: “Here’s a farmer, that hang’d
himself on th’ expectation of plenty: come in, time-server, have napkins enow about you;
here you’ll sweat for’t”. a farmer who hoarded corn expecting to make money, committed
suicide as the price of the crops dropped due to bounteous harvest. The porter asks him
to bring many hand kerchiefs to wipe away the sweat because the hell is very hot. The
porter imagines the second applicant for the entrance into hell to be a believer in
equivocation who can say yes and no to the same question to suit his purpose. But the
equivocation has not opened the gate of heaven i.e. pleased God, and he has to knock at
6the gate of Hell. The porter next, imagines the third knocker as the English tailor
come to heat his iron. Finally, the porter finds the place too cool for hell and says,
“I’ll devil porter it no further”.
This apparently disjointed, discordant and
drunken statement of the porter is sometimes criticized as unshakespearean. It is
considered spurious by Coleridge who declared emphatically that this low porter
soliloquy was written for the mob by some other hand, perhaps with Shakespeare’s
consent.  It gives Macbeth time to wash his hands and put on his night gown. There is
yet other who would justify the porter scene on the ground that this scene provides a
dramatic need of comic relief.
But De Quincey finds the scene all
Shakespearean but denies the part of comic relief. In fact, in his views it intensifies
the tragic impact in the play. He believes that both Lady Macbeth formed to ‘the image
of devils’. The next world is getting prepared for this message. In this intermingling
period, the porter appears in the scene. Like a great artistic skill here is the
hell-gate compared to Macbeth’s castle. The one a tipsy, tip soliciting menial whose
language is vulgar, whose jests are filthy but who after all is not a murdered; the
other, Macbeth, a valiant warrior speaking poetry.and yet a murderer. Thus the contrast
between the porter and his master is also established. The imagination of the porter is
also of hell minus tragic pangs, but a continuation of a tragic suspense. comic relief
from the tragic monotony.