Applying Tilak or Tika on the forehead is considered very auspicious in Sanatana dharma. 

In Hinduism, different sects, such as Shaivism, Vaishnavism, Shaktism wear different types of Tilak. 

Followers of Shiva wear Tripundra. 

Followers of Vishnu wear Urdhva Pundra. 

The human body is considered a temple of the lord. Thus Tilak is put to give reverence to the divine power within. 

Tilak is applied on the forehead between the brows. The most important energy center, the Ajna(Third eye) chakra, is situated here.

The Third eye chakra represents our subconscious mind and various thought processes. When tilak is applied, the negative energy is blocked from entering the body. 

Similarly, it prevents the positive energy from leaving the body.

Medically the joint also has a nerve (trigeminal) which when squeezed, clears sinuses. 

Massaging this spot by applying tilak cures insomnia, stress, and anxiety.