It is no error that Macbeth begins with the weird sisters. The evil that pervades the play is symbolized through their involvement.
Macbeth is essentially a tragedy but, unlike Shakespeare's other tragedies, there is
a supernatural dimension that purposively conspires against Macbeth
To retain a grasp on reality and to ensure that the audience recognizes the embodiement of evil, not only in the witches but in Macbeth himself, Shakespeare introduces the porter in Act II, iii. The audience can identify with the reality of a drunken porter and the tone of the play changes as the mood is lightened.
Macbeth is all about good versus evil, appearances being deceiving, ambition, choice and darkness in its literal and figurative sense. Even the drunken porter is aware - even subconsciously and certainly to bring an element of irony- that all is not as it seems and that sin and evil are commonplace.
He jokes that the "hell-gate" would be very busy with so many sinful people. His words about equivocation would have struck the audience of the day as
Shakespeare associates the use of equivocation by Elizabethan Catholics... with the words of the weird sisters.
The double meanings present in just about everything the witches say and their mantra "fair is foul" is reinforced by the inclusion of the porter. There is a clever, but subtle, foreboding in what the porter says as Macbeth will interpret the witches' words and contribute to his own undoing.
The assumed ramblings of a drunk are far more inciteful than may otherwise be assumed:
...it makes him, and it mars him; it sets him on, and it takes him off; it persuades him and it disheartens him...
The porter is therefore an essential part of the plot development.