Marinating in yogurt
The two general purposes of marinating is to tenderize and add flavor. What is happening is that the acidic elements are penetrating the meat and breaking down connective tissue. This causes the meat to become tender. Depending on your marinating liquid it also infuses flavors into the meat. I make a tandoori chicken recipe that calls for plain yogurt and spices. The flavor change will be minimal if you are using only plain unflavored yogurt. Here is an article that I found that goes into a little detail.  Marinade Science Article