The Untouchable concept of Brahmanas

The Brahmanas, at one point of time, even now in some parts of India used to follow a practice. The practice says nobody except a brahmana can, even by his or her shadows touch the brahmanas. Now question is about the correctness of the practice. To know whether the practice was correct or not, firstly we are required to know why was that practice made for the society to follow. Was there any practical reason for that? Secondly, as the effects were followed by the brahmanas, it is necessary to know whether the cause of the effect has been maintained or not, i.e. if there is a realistic cause for such a practice, do they still follow the cause for pursuing such practices.