I have almost forgot the taste of fears; The time has been, my senses would have cool'd To hear a night-shriek, and my fell of hair would at a dismal treatise rouse, and stir As life were in it; have supp'd full with horrors: Direness, familiar to my slaughterous thoughts, Cannot once start me."

The above's' question is: Why is the following comment by Macbeth in Act V, Scene 5 important?

Explanation:

That excerpt from Act V, Scene V, becomes important to keep in mind that how much Macbeth actually evolved throughout play. Macbeth hears a woman crying in this scene, and notes that the sound will have absolutely no impact on his feelings or nervousness.

Yet he remembers the time where he would have been unsettled and terrified by an unidentified noise just like this.

Macbeth accepts the cause for such a change: he has seen too many "horrors" in the way he used to, that he no longer feels terror. Those "horrors" are linked to his Scottish throne pursuit.

For example, the murdering of King Duncan, and his friend Banquo, and Macduff's family. He has engaged in too many violent violent acts that his reactions are not the same any more. He got desensitized to cruelty.

Furthermore, Macbeth's motivation has altered his personality almost beyond awareness and this excerpt shows that he is fully conscious of these alterations.