The main issue is that there needs to be a way to fine tune the tension in the system to allow for 'chain stretch. The previous answer is correct in so far as the need to have tension on the return run of chain, but an idler could allow for that.The horrible tension adjusters fitted on department store TSO's are, in fact, idlers in a sliding carrier. The *usually) extremely small diameter of the pulley wheel, and the overall lack of refinement of such designs makes it easy to overlook that this is actually an idler system at work.