Even if you're chopping perfectly vertically, a board will slide around or rock slightly. Like say you want to julienne some basil or something with a chef's knife, you'd probably be doing a rock chop, which involves lateral forces, causing the board to slide. Then also if you want to scoop up the food with the side of your knife, like scrape up minced garlic, those are lateral forces too. A wet cloth under a cutting board also softens/dampens the force of the knife hitting the board when chopping, which is nicer on your wrists.