It is believed that all Gods reside wihin the body of a cow
Since it is believed that, after death before heaven one reaches Vaitarni River, to cross this river one can hold the tail of cow & finally reach heaven
Cow dung & urine are said to have medicinal qualities
Since Lord Sri Krishna said in Bhagavad Gita "Amongst cows, I am Kamadhenu"
In Mahabharata it is said that whoever gives a cow in charity shall always be happy