There are many reasons why people feed yogurt to their chickens.  I personally have always fed it to them as a treat.  After much research, I have discovered the real benefits for feeding it to them, besides laughing at them while they enjoy eating it with it stuck all over their beaks.

The main reason:

 It can be used is to help cure sickness.

Yogurt contains Live Cultures. ( Probiotics )
Live cultures are live microbial feed suplements that affect the chicken ( or host ) by improving its intestinal microbial balance. Basically it helps keep the internal intestines healthy.

When chickens are sick and they are taking a antibiotic, it is advised you give them a teaspoon of plain yogurt a day.  Make sure the antibiotic allows dairy. ( You can do this by calling a feed store or the vet.)

The reason it is suggested to feed the yogurt is because the antibiotic kills off both bad and good bacteria in the chickens intestines. When you feed the chicken the yogurt, it helps to reinoculate its stomach with good bacteria and any remaining bacteria can not take over.

Yogurt Also Contains vitamin B-12

Vitamin B-12 helps the production and growth of red blood cells and bone marrow.

ALSO Contains:

Magnesium,Potassium and Calcium.

There are many good reasons to feed yogurt to your chickens.... 

* It helps heal them when they are sick.
*It contains calcium, which they need, so they can 
  produce egg shells.
*They love the taste of it!

* Note: If an antibiotic calls for no dairy, you can get a small bottle of Probis or another quality Probotic. It can be found at your local Feed store or Health Food store.