Why do I need a referral?
To get your insurance plan to pay for you to see that specialist. So first of all, make sure you understand what is covered under your health insurance plan. Put it this way: if you want to pay out of pocket, you can see whoever the heck you want. Just make sure to mention that to the receptionist when you are booking your appointment, so they can mark you down as self-pay, and they know they don't have to verify your insurance coverage for your appointment.
This is why it is so important for you, as the patient, to understand what is and is not covered by your health insurance plan. Some plans cost more for a reason!
Here's the general overview, though as health insurance plans keep proliferating, the best way to know for sure is to double-check with your specific insurance carrier:
Medicaid (MassHealth in Massachusetts) - You better make your PCP your best friend, because ain't no way you are seeing a specialist unless your PCP refers you to one, which they can only do after seeing you in person.
HMO (Health Maintenance Organization) - You DO need a referral from your PCP in order to see a specialist.
PPO (Preferred Provider Organization) - You DO NOT need a referral from your PCP to see a specialist, as long as your specialist is in-network with your insurance. Find out by asking the specialist's office when you call to make your appointment.
Medicare - You DO NOT need a referral from your PCP to see a specialist. You can make an appointment directly with the specialist's office.