What is the purpose or role of Lady Macduff in Macbeth?

Shakespeare liked to have a variety of characters in his
plays. In Macbeth he has one very strong female role in Lady
Macbeth, but there are no other good female parts. The three witches are putatively
female but supernatural creatures and not real women. Banquo says to them, in one of the
rare touches of humor in this grim play:

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You should be women,
And yet your
beards forbid me to interpret
That you are
so.

Shakespeare may have
invented a scene for Lady Macduff to try to give his play a little more balance or
variety. The audience has to be able to tell characters apart as they come and go on the
stage, and this is a major reason that plays typically feature young and old, male and
female, and contrasting types. The scene in which Lady Macduff appears is not really
necessary, since Macduff is informed that his castle was stormed and his whole family
slaughtered.

Shakespeare must have had permanent members of
his troupe who specialized in female roles and may have wanted to give one of them
something to do. Shakespeare was not only a writer but an actor, a director, a producer,
and a co-owner of the theater. The same youth who played Lady Macduff may have changed
clothes and appeared as the Messenger who reports to Macbeth in Act
5:

As I did
stand my watch upon the hill,
I looked toward Birnam, and anon
methought
The wood began to
move.

In many of
Shakespeare's other plays it is obvious that he liked plots that would include a variety
of characters, especially including females. In King Lear he has
three strong roles for Goneril, Regan, and Cordelia. In Othello he
has three good parts for Desdemona, Emilia, and Bianca. In Hamlet
he has good roles for Gertrude and Ophelia. Even in Julius Caesar
he includes roles from Calpurnia and Portia in order to keep his play from seeming like
nothing but a lot of middle-aged men strutting around in togas and hard to tell one from
the other. In As You Like It, Shakespeare has good roles for
Rosalind, Celia, Phebe and Audrey.

This is called
"orchestration." In a modern play like A Streetcar Named Desire,
you can see how the author developed a plot that would offer good parts for a variety of
characters--Stanley and Stella Kowalski, Blanche du Bois, and Harold
Mitchell.

In Macbeth, however,
although he had one powerful female role, the play was otherwise overfreighted with
military men. Perhaps too much importance has been attributed to Lady Macduff. She may
exist in the play mainly to add just another female role and provide a little visual and
even audio variation. There is also a brief role for the Gentlewoman in Act 5, Scene 1.
This "woman" may be the lad who played Lady Macduff earlier, now dressed in a different
garb.