Macbeth was interested in Banquo’s travel plans because he is planning to kill him and his son Fleance.
Macbeth is worried that Banquo is going to tell someone about the witches’ prophecy, or that Banquo suspects him of murdering Duncan. He is also worried that Banquo might become king, because the prophecies said his sons would be kings.
To be thus is nothing,
But to be safely thus. Our fears in Banquo
Stick deep, and in his royalty of nature
Reigns that which would be fear'd. ’Tis much he dares, (Act 3, Scene 1)
Macbeth wants to make sure that he knows where Banquo is, because he is sending three murderers after him and his son. With Banquo out of the way, and Fleance dead, Macbeth will be able to be “safely” in his throne.
Even though Macbeth and Banquo were good friends, once they meet the witches Banquo begins to worry about Macbeth, and Macbeth begins to want to eliminate the threat that is Banquo.