Belts have to make at least a 180° wrap around engine components. To do that, they sometimes have to place an "idler roller" under the component before they can route the belt to the next component. That idler roller doesn't power anything. It just keeps the belts wrapped properly around the preceding pulley. The roller has a bearing in it and sometimes that bearing wears out.
Rick
I'm a retired ASE Master/L-1 Technician. I still keep current with the latest automotive technology. Visit my blog for cool articles and TSB's: http://free-auto-repair-advice.blogspot.com