The tilak invokes a feeling of sanctity in the wearer and others. It is recognized as a religious mark. Its form and color vary according to one’s caste, religious sect or the form of the Lord worshiped.
The vaishya wore a yellow kesar or turmeric mark signifying prosperity as he was a businessman or trader devoted to the creation of wealth.
The sudra applied a black bhasma, Kasturi or charcoal mark signifying service as he supported the work of the other three divisions.
It is known as the Aajna Chakra in the language of Yoga. The tilak is applied with the prayer - "May I remember the Lord. May this pious feeling pervade all my activities. May I be righteous in my deeds.”
Even when we temporarily forget this prayerful attitude the mark on another reminds us of our resolve. The tilak is thus a blessing of the Lord and a protection against wrong tendencies and forces.
The entire body emanates energy in the form of electromagnetic waves - the forehead and the subtle spot between the eyebrows especially so.
That is why worry generates heat and causes a headache. The tilak and pottu cool the forehead, protects us and prevents energy loss.
Sometimes the entire forehead is covered with Chandan or Bhasma. Using plastic reusable "stick bindis" is not very beneficial, even though it serves the purpose of decoration.