Final answer:
Macbeth lies to Banquo to conceal his ambitious thoughts influenced by the witches' prophecy and to avoid suspicion as he plots to gain power.
Step-by-step explanation:
Macbeth lies to Banquo about the witches because he is driven by ambition and the desire to become king. After the witches prophesy that Macbeth will be king, he becomes consumed with the thoughts of gaining power. He lies to Banquo to cover up his growing obsession and to prevent Banquo from suspecting his dark aspirations. This deceit is a part of Macbeth's larger pattern of manipulation and treachery that ultimately leads to his downfall.