In Act 3, why does Macbeth invoke the night?

I'm gathering you're talking about Act 3 Scene 2:

Come, seeling night,
Scarf up the tender eye of pitiful day;
And with thy bloody and invisible hand
Cancel and tear to pieces that great bond
Which keeps me pale!--Light thickens; and the crow
Makes wing to the rooky wood:
Good things of day begin to droop and drowse;
Whiles night's black agents to their preys do rouse.--
Thou marvell'st at my words: but hold thee still;
Things bad begun make strong themselves by ill:

 Night represents moral darkness, the spirits of darkness, and concealment. Macbeth is preparing to have Banquo murdered, and, just as Lady Macbeth earlier wished for the spirits to "unsex" her, Macbeth wishes for night to cover any possibility of pity (scarf up the tender night of pitiful day) and to give him the concealment necessary to do his deed without being caught. There's power in the darkness, both literally, and, for Macbeth, figuratively. By now he has completely crossed the line and he no longer wants to be in the sun.