The red dot is called a bindi. It is a form of body decoration. Culturally, hindus attach a lot of importance to this ornamental mark and call it by various names - pottu, bindi, tikka, tilakam and sindoor. There are various theories on why it is important and I will share a few here.
why should a girl put a bindhi as per tradition
It signifies a point of awakening and is usually activated while pursing spiritual sciences. In Chinese acupressure sciences, this point is considered to be a converging point for nerves and a massage is believed to relieve headaches. Ayurveda also believes massaging this spot will relieve stress and insomnia.
The 'Magic' Ingredient - Traditionally kumkum ( a mixture of turmeric and lime) or chandan (sandalwood) was used to mark a bindi. Kumkum is hygroscopic in nature and can greatly assist to remove excess water from one's head. Tumeric's anti-bacterial properties are helpful too.
Further, the colour red is believed to aid spiritually. Chandan or sandalwood is renowned for its 'cooling' properties. For a hot tropic country, application of chandan on forehead ( a nerve centre) helps to cool one's entire system.
On a religious note, the color red symoblises 'shakti', the goddess of strength. A kumkum is marked on girls after marriage denoting strength and wishing for peace.