These days it's getting harder and harder to see a specialist. If you have preferred provider or health maintenance organization insurance you will often find it necessary to ask for referral to a specialist. Some plans have specialists on their panel, others don't. If there is no specialist on the panel you will have to ask for outside referral.
Some plans are happy to oblige.
Others make it difficult. You may have to work hard to get a referral and you might need to pay extra money for seeing a doctor outside the plan. Medicine is still an art, not a science. Experience and wisdom play a major role in making the right choices and getting the best results. It's true that every doctor graduated from medical school.