Onomatopoeia can in my opinion really add something to a story.

The verbal description, "He slapped her ass again and again as the tears ran down her face" isn't nearly as powerful as "The loud smack of his hand as it landed on her ass again and again drown out the soft spattering noise of her tears falling onto the floor." The sentences are not exactly equal but they are pretty close, and with the addition of the "smack" and "spattering" I really get more of a feel for what the story is trying to convey.

Drake.