Citrus fruits contain quite a lot of vitamins and minerals in them, some of which are:

• Vitamin C, also known as Ascorbic acid and is well for its antioxidant activity.
  Citrus fruits are known to be quite rich in Vitamin C.
  Vitamin C is also known to help fight off diseases such as the common cold, cardiovascular disease, diabetes, hypertension, prevents scurvy and also helps to heal wounds.
  It also helps to eradicate free radicals present in the skin, also free radicals can cause cataract, which is a degenerative eye disorder.
  And last and not least it helps to enhance the immunity of the body.
  
• Folate, also known as vitamin B9.
  Citrus fruits are moderately rich in folate.
  Folate is known to prevent anemia and help prevent cancer.
  Also folate is known to help produce serotonin which helps to make your mood better.
  
• Vitamin D, Citrus fruits are moderately rich in Vitamin D.
  Vitamin D is known to prevent rickets and prevent cancer and many other diseases.
  Vitamin D also helps to absorb calcium in the body.
  
• Calcium, Citrus fruits are moderately rich in calcium.
  Calcium is needed to help the bones grow and to keep them strong.
  
• There are many other vitamins and minerals present in citrus fruits such as selenium, thiamine, niacin, magnesium, copper, potassium, iron, manganese, zinc, phosphorus, vitamin E which have many health benefits as well as preventing many types of cancer.
  
• Also they are known to aid in weight loss, A cup of lemon juice or lime juice will help in weight loss.
  

These are just some of the many health benefits.
Also they have always been used since olden times in Europe, China, India to cure and then by the British navy to prevent and cure scurvy on ships and thus the British were called the limey.
And even now they are still being used for many medical benefits in the modern world.