When you desrcibe something in a passage, it helps the reader understand and visualize what happened or what is happening. For example, when Equanio talked about the fish he described them jumping and flying across the ship. With that, the readers can see what is happening inside theiir mind. This makes them more attracted to reading the excerpt even more, and gives them a good understanding about the setting of the story.
During our passage, I first saw flying fishes, which surprised me very much; they used frequently to fly across the ship, and many of them fell on the deck. I also now first saw the use of the quadrant; I had often with astonishment seen the mariners make observations with it, and I could not think what it meant. They at last took notice of my surprise; and one of them, willing to increase it, as well as to gratify my curiosity, made me one day look through it. The clouds appeared to me to be land, which disappeared as they passed along. This heightened my wonder; and I was now more persuaded than ever, that I was in another world, and that every thing about me was magic
