The black beads in it are meant to ward away the evil eye from the couple. According to traditional beliefs beads absorb the negative energies which are focussed toward the couple even it radiates only happiness for their life together. Its also believed that these sacred black beads safeguard and protect the life of the brides husband.
The importance laid on this auspicious thread is due to the belief of people who believe in it with that mentioning about it in the ancient Hindu scriptures also put an effect on people mind. The mangalsutra are not just worn by the Hindus brides, but its been worn by the brides of many other states due to its auspiciousness