Why are these lines from act i important in the plot of the play? macbeth. [aside.] glamis, and thane of cawdor: the greatest is behind.— for your pains.—do you not hope your children shall be kings, when those that gave the thane of cawdor to me promis'd no less to them? banquo. that, trusted home, might yet enkindle you unto the crown, besides the thane of cawdor. but 'tis strange: and oftentimes to win us to our harm, the instruments of darkness tell us truths; win us with honest trifles, to betray's in deepest consequence.—

Macbeth has just found out he is the Thane of Cawdor from Ross and Angus, making the second part of the witches' prophecy true. This causes him to really wonder if he's destined to be king. This idea gets the best of him, and he convinces himself that if he'll become king anyways, he might as well kill the king himself. This thinking will lead to him committing the murder, leading him down a path of destruction. 

Banquo questions that perhaps the witches aren't telling the truth after all, offering Macbeth a sense as reason. He knows that witches will sometimes tell part of the truth to earn their trust, but incorporate lies that will cause chaos because that is the witches' nature. But this proves to be a useless warning.