The significance of Banquo's speech in Act 3.1 of Shakespeare's Macbeth is twofold.
First, the speech establishes that Banquo suspects Macbeth of treachery. Macbeth, Banquo says, now has it all: Cawdor and the crown. And Banquo suspects that he played treacherously to get it. In other words, he suspects Macbeth assassinated Duncan to get the throne.
The speech also continues the idea of Banquo as a foil to Macbeth. Banquo would like the witches' predictions to come true, too. He would like his heirs to rule Scotland. He has interest in the predictions the same as Macbeth does. But he doesn't kill anyone to make the predictions come true. He handles the situation as one should.
He warns Macbeth earlier in the play that sometimes forces of evil give men a little truth, in order to earn their trust so they can bring them down later. Macbeth totally ignores Banquo's warning. Macbeth's ambition gets in the way of caution and truth. Banquo's does not.