Why do we hear about Macbeth from others before we meet him? What is the effect?

The fact that we first hear of Macbeth's valor and courage from the wounded captain allows us to make our own judgments about what we hear. Admittedly, the captains accolades leave us with a positive view of Macbeth as a man and soldier. None-the-less, the character we see through the words of another man are a far cry from what we see when Macbeth is first introduced. Macbeth's interaction with the witches makes us question our own gullibility, as we see a blatantly ambitious man with a huge lack of self-confidence. Macbeth's weak character and inner turmoil will wreak havoc on him for the duration of the play.