The three witches have a very important role to play in Macbeth. They have to be introduced and identified as witches, whether it is to a modern audience or an Elizabethan audience. Shakespeare chose to introduce them separately. He could hardly have them suddenly appear to Macbeth and Banquo and foretell their destinies while introducing themselves at the same time. Good writers usually introduce important characters separately wherever possible. Lady Macbeth, King Duncan, and Macduff are all introduced separately. We have to know who these actors are supposed to be. The opening scene also has shock value. Shakespeare always needed to quiet his audience, especially those standing in the pit, so that the subsequent dialogue could be heard.
The opening scene tells us that we will see these strange creatures again.
When shall we three meet again?
In thunder, lightning, or in rain?
It also informs us that they must have something to reveal to the character whose name provides the title of the play.
There to meet with Macbeth.
And their brief exchanges of dialogue inform us that there is a battle taking place somewhere nearby which will be settled before sunset.
When the hurlyburly's done;
When the battle's lost and won.
That will be ere the set of sun.
These strange-looking and strange-sounding creatures have silenced the audience, whetted their curiosity, and indicated that "they have more in them than mortal knowledge," to quote from Macbeth's letter to his wife. A modern audience may not be as impressed by spooky-looking characters, since the movies have inured us to much worse sights than a few witches, but we will still be anticipating their imminent meeting with Macbeth, during which we hope to learn what this play is really all about.
Modern audiences might benefit in any number of ways from seeing this work. A modern interpretation of Shakespeare (done with contemporary or modern costumes and sets, for example, set in a later time period than the play's origins) often allows for insights and connections to modern audiences, since Shakespeare's plays contain such timely and universal themes. Certainly, the opening scene of the Scottish play is one of Shakespeare's most memorable and unusual theatrical constructions. The three witches are mysterious, and somewhat threatening (although depending upon the visuals and sounds employed, they could be made to seem less so).
The ending lines, "Fair is foul, and foul is fair" refer ostensibly to the weather, as the stage directions state there is thunder and lightning, and the witches also mention the weather when discussing their next meeting. But, this wordplay may also allude to the grey areas of morality and ethics, and the notion that any action can be justified if the actor believes it is necessary. Certainly this is a major theme in the play of the justification of murder.
The witches also refer to the battle being "lost and won." A modern audience might find resonance with these lines and this idea, as we witness acts of war and other actions that cause harm and destruction. Depending upon the players involved, such acts may be recognized as heinous, but are carried out because there is a bigger picture that finds such actions to be necessary and justifiable. This same justification governs Macbeth's actions, and his wife encourages him.