After the banquet scene in Act III, Macbeth tells his wife:
I will tomorrow, / And betimes I will, to the Weird Sisters: / More shall they speak, for now I am bent to know / By the worst means, the worst. For my own good, / All causes shall give way.
Literally translated, Macbeth says that he intends to visit the witches in order to hear the worst news from the worst sources. Everything else is secondary.
Macbeth is worried about what others think of him based on his public reactions to Banquo's ghost. Now that the supernatural is discernable to him, he wants to consult his own supernatural sources to see what his future holds.
Macbeth is on the run here, and he knows Banquo's ghost is an omen of bad things to come; time and the armies of Macduff and Malcolm are aligned against him. But he has secret hole cards: the witches' knowledge of the future. They have already correctly predicted his future (in Act I). Now, Macbeth wants to hear "how bad it is going to be" in order to prepare against it.