Another scenario is a series of rollers, such as used for pressing paper. Each roller has to be powered, but adding a motor to each one is wasteful (and it can be difficult to synchronize rotational speed with independent drive systems). One could simply add a gear onto the end of the shaft of each roller, but that means that each roller would be spinning the opposite direction of the one before (and therefore rubbing against each other as the turn). By simply adding a small idler gear between each larger gear, the result is a series of rollers, all being powered in the same direction.