What is the benefit to a doctor to be "In Network" with an insurance company?

A doctor contracts with insurance companies to accept a lesser payment than what he would normally charge. What is the benefit to the doctor to do that?What is the benefit to a doctor to be "In Network" with an insurance company?
They get a lot more clients through network referrals.
They have patients steered their way, from the insurance company. It's an indirect form of marketing.What is the benefit to a doctor to be "In Network" with an insurance company?
The doctor gets more patients. Patients who cannot afford the out-of-network copays will only go to in-network doctors.
One of the biggest benefits is stream-lined payments. Because the doctor agrees to only charge specific rates they get the benefit of getting the billings handled very quickly.

If they were outside of a network they would have to bill their customers directly then have to deal with all of the headaches that come from that. The billing coupled with "free" marketing is a big win for a small doctor's office.What is the benefit to a doctor to be "In Network" with an insurance company?
They get volume because they are listed as an accepted provider when people search for a new doctor. There are plenty of doctors who don't take any insurance at all, they are just very expensive instead.