While the ads the user initially sees may be relevant to a publisher's content, they may not be precisely what the user is seeking -- for example, a user may see ads about cheddar and brie cheese but would prefer more information about Swiss cheese. With the 'next' and 'previous' buttons, users can view more cheese ads until a Swiss cheese ad appears.
You won't generate earnings for clicks on the 'next' and 'previous' buttons, but these buttons will help improve both advertiser value and your potential revenue. When users click on the buttons, they begin interacting with the ads and are more likely to find the specific offering they're looking for, which can lead to higher earnings for you.
