The skin is a very important body organ, and a wide range of conditions can involve the health of the skin. A dermatologist can examine problem areas of skin to determine what is causing the problem, and recommend an appropriate course of action. For many conditions, this might involve a topical medication, or an oral medication.
For something such as suspicious looking moles or skin cancer screening, the dermatologist might take a biopsy to find out whether or not the mole or lesion is malignant.