Macbeth has become intimate with the powers of darkness through the course of the play. He first invoked the power of darkness in Act 1, sc. 4, right after Duncan has named his son Malcolm as the Prince of Cumberland to succeed him to the throne. Macbeth says, in an aside, "Stars, hide your fires / Let not light see my black and deep desires." Darkness literally and figuratively hides many deeds that Macbeth and Lady Macbeth do not want seen. Darkness also represents evil which grows in intensity in the play. Macbeth has Banquo killed because he fears that Banquo may suspect he had something to do with Duncan's murder but he also has Banquo killed because he is jealous of Banquo. The witches told Banquo that his sons would be kings and Macbeth hopes to stop that by killing both Banquo and his son, Fleance. These are evil acts and Macbeth does not want the light of truth or the light of day to reveal them for what they are - simple murder.
Macbeth invokes the night because he has engaged murderers to kill Banquo and Fleance. So he feels that when nightfall comes he will be free of the threat of Banquo and his heirs.
He has come to appreciate the night and the evil that can be wrought and how quickly, in the dark, a problem is solved, for Macbeth of course. The night, which has an association with the forces of darkness, which Macbeth has now become intimately linked, for now will satisfy his fear of threats to his crown. When night comes, and Banquo and Fleance are killed, never to pose a threat or raise a suspicion, Macbeth believes that he will be free to enjoy being king.
In the darkness, Macbeth takes care of his problems, when he thinks that no one can see, but he is deluding himself.