The figurative language and word choice are what make this such a famous, classic poem. Alfred Noyes uses fantastic, vivid visual imagery in "The Highwayman," and each phrase is infused with multiple literary devices. For example, by describing the moon as a "ghostly galleon tossed upon cloudy seas," it provides a feeling of mystery, suspense, and general spookiness, which is fitting as the poem ends as a ghost story.
Noyes could have written, "It was a darky, windy night. The moon was out. There was a curved road through a field." Contrast that with the actual figurative language he used:
The wind was a torrent of darkness among the gusty trees.
The moon was a ghostly galleon tossed upon cloudy seas.
The road was a ribbon of moonlight over the purple moor.
His pistol butts a-twinkle, his rapier hilt a-twinkle under the jeweled sky.
Noyes could have written that there were stars out that night. However, by referring to the sky as "jeweled," Noyes suggests that the stars are not only out, but are sparkling and twinkling like jewels in the sky. It is a much more powerful visual image. One can picture the clear night, the bright stars twinkling against a velvety, black backdrop.