Why did Macbeth help the king defeat the rebels and Norwegians?
Under the prevailing feudal system, Macbeth derives his title as Thane of Glamis, and the lands and privileges that go with it, from the king, who is absolute monarch. The same applies to all the other thanes. They support the king, and the king legalizes their titles and lands. The people who live on the lands are serfs of the thanes and have to work for them and pay them taxes. The thanes can conscript serfs to fight in their armies. Macbeth is obligated to fight for the king whenever called upon, bringing his soldiers with him. In this recent war, the king rewards Macbeth by naming him Thane of Cawdor, after ordering the execution of the traitorous Thane of Cawdor in Act I, Scene 2.
No more that Thane of Cawdor shall deceive
Our bosom interest. Go pronounce his present death,
And with his former title greet Macbeth.
(Note how Shakespeare's scenes often end with rhymed couplets. This seems intended to remind the audience that they are hearing poetry, usually in unrhymed iambic pentameter. The poetry is by far the most important element of Shakespeare's plays.)
The title of Thane of Cawdor enriches Macbeth greatly because of the ownership of the lands that go with it and virtual ownership of the peasants who have to live on the land. The king and the thanes ruled by force. No one dared question their rights to dominate and exploit the peasantry. That would be treason. Obviously, the king has the power to bestow titles and also the power to revoke them. Macbeth must obey Duncan implicitly. In Act I, Scene 4, Macbeth tells Duncan:
The service and the loyalty I owe,
In doing it, pays itself. Your highness’ part
Is to receive our duties, and our duties
Are to your throne and state, children and servants,
Which do but what they should, by doing every thing
Safe toward your love and honor.