To the question: "What is he?", the answer can be "He is married, tall, doctor in medicine".
On another point of view, "He is a doctor" = "one in the set of the doctors". (But which one of the crowd there? The tall one is the doctor.)
"He is doctor in medicine": identity; "He is a doctor": aside property.
With different expressions, you are not expressing the same thing, but different ideas (some of them more preponderant in use in a territory).