Question

What vivid image, typical of Elizabethan drama, does Shakespeare create when Macbeth says to Ross, “...why do you dress me/ in borrowed robes?”

Answer 1

The central topic in this scene is clothing. When King Duncan grants Macbeth with thane of Cawdor, Macbeth refers to this title as "borrowed robes" because thane is alive and he doesn't want to wear the old man's clothing. Clothing is a symbolism for rank here representing the status in a community, in this scene in a royal one. Macbeth doesn't want the rank while the holder is still alive.        

Answer 2

In that line, Macbeth is comparing the strange, good news of his promotion to wearing fancy clothes that a person of high esteem would naturally wear. The setting of the play is the 11th century, so the king, and very likely his thanes, would certainly wear noble clothing including (sometimes) elegant robes.