While PBMs market themselves as being well-positioned to bring savings to plans and consumers, a lack of transparency in their practices enables them to wield their power to increase their profits, often at the expense of the consumer.
The National Community Pharmacists Association prepared a presentation (available here) detailing many common PBM practices that drive up health care costs. Some of the more prominent examples are:
How Transparent PBMs can Save Money:
Here is a resource published by PBS on how PBMs keep prescription costs high for patients.