Question

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.—Thanks 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.—