Why would one have to see a board certified physician of a particular specialty; can md's work in a field without the certification?

Yes .: Of course. Most doctors are board eligible and can practice as well without passing their specialty boards although most hospitals now prefer board certified physicians. Board certification is a verification that they are competent in their specialty.

Your choice: States license physicians to practice medicine. There is an expectation that they will treat cases that they were trained to do, but this is not always the case. They might enter practice right out of medical school or with a single year of internship and then offer to treat anything that comes in. For some issues that is fine, but lack of specialty training can lead to delays or wrong treatment.