Act 4 Scene 2 of Macbeth is the longest scene in the play. Yet neither of the protagonists appear in it. Why is important in this scene?Why is it included?
Concerning Shakespeare's Macbeth, Act 4.2 is not a long scene at all. It shows Lady Macduff and her son being killed. It's vital because it makes Macbeth's order to kill Macduff's family visible. It concretely shows the murders occurring. It's also important for other reasons, but I think you probably meant to say Act 4.3, so I'm going to move on to that.
Act 4.3 shows Malcolm testing Macduff to see if he's loyal to Scotland or to Macbeth. It's long because the tests Malcolm uses are intricate and complicated and thorough. Malcolm pretends to be lustful, greedy, and an all-around terrible human being. He waits to see whether or not Macduff will keep saying "Oh, it's all right, you can still be king" (I'm paraphrasing), or if he will turn away in disgust. If Macduff turns away in disgust then Malcolm will know he has Scotland's best interests at heart, and is not just playing along in order to betray him to Macbeth. That's what happens, of course.
Also, this scene includes Macduff finding out that his family has been killed.
This scene furthers the plot (Malcolm and Macduff join forces to lead an army against Macbeth), and reinforces themes (fair is foul and foul is fair--Malcolm suspects Macduff is foul, but he is really fair; and role reversal--Macduff must mourn before he seeks revenge, which is typically something more expected of a female).
By the way, the scene also pictures the English King Edward, so devout that he heals people. His character is a stark contrast to Macbeth's.
I think that you are talking about Act IV, Scene 3, not Scene 2. Scene 3 is the longest in the play and Scene 2 has the murder of Macbeth's family, which is a pretty important part of the plot. So I will talk about Scene 3.
The reason that this scene is important, to me, is that it shows what is wrong with Macbeth both as a king and as a man. As a king, he is selfish. We are alerted to this by the way that real kings (Duncan and Edward) act and talk. They care about their people and their kingdoms.
As a man, Macbeth is pretty much just violent -- he is aggressive and murderous (at least in the play). Macduff's reaction to his family's death is more than this. He is angry, but he also has feelings of grief. Real men are like that.
So the scene shows us what real men and real kings are like. This points to how Macbeth is different from that idea of manhood and kingship.