Can anyone explain why coverage networks even exist? What exactly makes it so expensive for an insurance provider to deal with an out-of-network health care pro

Can anyone explain why coverage networks even exist? What exactly makes it so expensive for an insurance provider to deal with an out-of-network health care provider? That same health care provider would not be expensive for an in-network insurance provider. I have no sense whatsoever for why the same health care provider would be expensive in one case and inexpensive in another case.