To answer this question, take a look at the conversation between Macbeth and Lady Macbeth in act III, scene IV. Pay particular attention to the following lines:
For now I am bent to know,
By the worst means, the worst. For mine own good,
All causes shall give way.
In other words, Macbeth is desperate to know what will happen in the future. Clearly, he is feeling insecure about his reign. Specifically, he is worried about Macduff and why he did not attend the banquet. He has also just seen the ghost of Banquo, so perhaps feels that Banquo's sons still pose a threat to him.
The fact that Macbeth seeks the witches shows just how much he trusts them. He believes wholeheartedly that they can give him the answers that he seeks. As Macbeth says in this quote, he wants to know his future. He does not care if the future looks good or bad—he just wants the chance to prepare himself accordingly.