Like so much in Hinduism, there is complexity in the answer. On one hand, I think that the use of the bindhi is used to cover or protect the "third eye" of insight within the soul. This notion of the "third eye" is predominant in the Hindu religion. Lord Shiva's third eye, located at his forehead, is capable of intensity and power, as is the same in the Goddess Durga, and Lord Ganesh. All Hindu deities possess the third eye and this is something within human beings, a realm that "leads to inner realms and spaces of higher consciousnesses." The bindhi is meant to protect this third eye, a form of intense power on a spiritual level. For example, it is Lord Shiva's third eye that incinerates Lord Kama who wishes to distract him while he is meditating and it is Lord Shiva's third eye that causes the hands of Goddess Parvati to perspire while she attempts to playfully cover his third eye. The use of the bindhi is to cover this third eye, and is something to protect this realm or portal to higher consciousness. Akin to so much in modern Hinduism and the globalized world, the bindhi has become a fashion statement, something that is able to merge the spiritual element with a secular one.
I cannot ignore the views put forward. But the bindi is arguably the most visually fascinating of all forms of body decoration. Hindus attach great importance to this ornamental mark on the forehead between the two eyebrows -- a spot considered a major nerve point in human body since ancient times. Also loosely known as 'tika', 'pottu', 'sindoor', 'tilak', 'tilakam', and 'kumkum', a bindi is usually a small or a big eye-catching round mark made on the forehead as adornment.
In India it is the prerogative of the married woman. A red dot on the forehead is an auspicious sign of marriage and guarantees the social status and sanctity of the institution of marriage.