The agenda is not fixed. In our case, with a Blue Cross plan, the case manager was instrumental in expediting our wheelchair order given an extremely indifferent DME, and getting the seat elevation feature fully reimbursed. A benign reason for the payer [I worked for one] to have case management is that admin costs, back and forth, are lower when complex people who will otherwise contact the general call center, have a specific person to call.
The case manager can help herd cats, avoid duplication and flag discrepancies that delay treatment/reimbursement.That is why I suggest everyone not getting what they want when they want it check it out. It can be a shorter distance between two points.
The case manager does not pretend to represent any treating clinicians, though s/he may interact with them. But like I said above, just like all docs aren't created equal, neither are case managers.