Why did the weird sisters creating surprise Macbeth and banquo

Ibelieve this is the part where ulysses reveals his true identity to his faithful servants eumaeus and philaetius: his ragged vest then drawn aside disclosed the sign conspicuous, and the scar exposed: eager they view'd, with joy they stood amazed with tearful eyes o'er all their master gazed: these servants know ulysses by the scar he got when he was a child - so when he showed them the mark, they immediately knew this is truly their master and they hugged him, finally starting to believe his words.