Why is Lady Macbeth walking in her sleep a significant event in Macbeth?
I would argue that this is a significant event for the following three reasons. Firstly, it acts as a proof of what has only been rumour before that. The way that Lady Macbeth sleepwalks in front of the Doctor and Gentlewoman and obviously confesses her own involvement and her husband's involvement in Dunan's murder gives tangible proof of Macbeth's crimes and tyranny, which of course matches what is happening in Scotland. Note what the Doctor says having seen Lady Macbeth and heard her words:
Foul whisp'rings are abroad. Unnatural deeds
Do breed unnatural troubles: infected minds
To their deaf pillows will discharge their secrets.
The Doctor sees this act of evil, the murder of Duncan, as the cause of "foul whisp'rings" and the "infected" mind of Lady Macbeth is the result of her "unnatural deeds."
This of course leads us to the second way that this is important, which is to show the disintegration of Lady Macbeth's character. In Act One and the murder of Duncan, we see that she is the force to be reckoned with. It is she that goads her husband into committing regicide and she that willingly abandons herself to the forces of evil so her husband can become king. Yet here we see the result of those actions. She is obviously suffering the results of her "unnatural deeds" and the evil she has committed.
Lastly, let us just briefly think about how this impacts Macbeth. From this scene in Act V scene 1 we can see that there is obviously a split growing between Macbeth and his wife, which has been shown through the way that he has orchestrated further murders without her help. Now his wife is clearly deranged and we can see how Macbeth is becoming further isolated. Even his wife is no longer able to support him.