Banquo wishes to know what the wild and withered creatures are that he is seeing, which suggests that the witches have an unkempt look and appear very old. This further indicates that they look like savages, unbound and uncivilized, and that they seem like a gnarled tree or plant, devoid of any vitality. Banquo, furthermore, rhetorically ask why they look like supernatural beings which should not be be earthly bound but appear on it anyway.
He wants to know whether these entities are alive or if they are of such essence that they cannot be questioned - their appearance and existence should be taken for what it is. This further suggests some kind of supernatural force. In trying to make sense of these wretched creatures, Banquo states that they seem to understand him since each one of the three has placed a gnarled finger to its lips, instructing him to be silent and not ask questions.
Banquo suggests that they seem to be women but the fact that they have beards prohibit him from making such a call.
Banquo's remarks fit in with the theme of appearance and reality - what you see is not entirely the truth. Things may appear a certain way but are, in reality, quite different from what they seem. This is made clear in, for example, king Duncan's earlier assessment of the thane of Cawdor, whom he trusted. He was betrayed by the thane who assisted the king of Norway and the traitor, Macdonwald, to rebel against him.
Further examples of the theme is when Lady Macbeth asks to be 'unsexed' and when she tells her husband to 'look like the innocent flower, but be the serpent under't. It is also found in the witches' paradoxical and unequivocal statements such as 'fair is foul and foul is fair' and in Macbeth's ultimate betrayal, killing his king who had been both a relative and patron to him.
There are many other examples of the theme throughout the play and it is this that enables the witches, Macbeth, and his wife to fool and mislead others and exercise their malicious intent. The allusion to savagery in Banquo's assessment of the witches also foreshadows the savage rule that Macbeth's tyranny will introduce - Scotland will be plunged into chaos in which his ruthless bloodthirst will be the order of the day until peace is restored with his assassination by Macduff.
Banquo describes the witches as he says:
What are these
So wither'd, and so wild in their attire,(50)
That look not like the inhabitants o’ the earth,
And yet are on't? Live you? or are you aught
That man may question? You seem to understand me,
By each at once her choppy finger laying
Upon her skinny lips. You should be women,(55)
And yet your beards forbid me to interpret
That you are so.
This fits into the larger themes of the play because the play is about the withering of the heart and the wildness that fester in the MacBeth's psyches.
Another key theme in MacBeth is that of the questioning man. MacBeth questions the authority, questions himself, and even Lady MacBeth questions him.
The theme of disguise and loose fitting clothes seem appropriate here also. MacBeth wears the robes not meant for him, just as the beards are not really meant for women.