Why do Hindus wear religious marks?

[pramanisa]

Wearing religious marks ( also called Tilak) is a custom followed by Hindus. It mainly invokes a feeling of sanctity by the wearer and the onlookers as well. Religious marks are worn by men and women with ashes, clay, kumkum (Powdered red turmeric) or sandalwood powder. It is a visible sign of a person as belonging to Hindu culture.The Tilak is also believed to have medicinal and protective functions. The pastes applied are considered "cooling", and are applied to the ajna chakra, a concentration of spiritual energy on the forehead between the eyebrows. The Tilak is also considered to bestow spiritual comfort and protection against demons, bad luck, and other evil forces.

Saivites typically use ashes ( called Vibhuti) and draw their tilaks as three horizontal lines ( tripundra). Vaishnavites apply clay (preferably from holy rivers) or sandalwood paste. They apply the material in two vertical lines, which may be connected at the bottom, forming either a simple U shape or a form said to be like a tulasi leaf. Their Tilak is called the urdhva-pundra.

Vibhuti used by Saivites, means glory and it is also called bhasma (that by which our sins are destroyed and the Lord is remembered ). The holy ash is worn with adoration and respect. This is also known as “ thiru neeru” in Tamil. The holy Ash has lots of spiritual meaning. There are many hymns praising the glory of the holy Ash and one popular one is “Mandiramaavathu neeru”. Vibhuti is so named because it endows one wih prosperity.
Ash is the substance that results when things are completely burnt off. In natural terms it is a final state. It is also known as Bhasma because it burns away all sins. This ash is the ultimate reality and cannot be changed any more. By applying this as a symbol of Divinity, we prepare ourselves to give up all desires, burn our attachments and temptations and make ourselves pure, holy and sacred, for liberation.

Vaishnavites use clay for their Srichurnam. This is also called “ thirumann” ( mann is the tamil word for clay) and the shape resembles the lotus feet of the Lord. This is known as Srichurnam and wearing this is as an important part of the daily rites of a Sri Vaishnavite. The Tilak is applied to twelve parts of the body, reciting the twelve names of the Lord. Sri represents the permanence of a Jiva wedded to the Lord. Vedas say, by wearing this mark, he becomes fortunate, gets released of all the worldly bondages and attains liberation. In Sri Vaishnava sampradaya the tilak is made out of the white mud found in anthills. The scriptures tell us that the mud from the base of a Tulasi plant and the white mud from within the anthill are both pure and best for making tilak. The Sri Vaishnavas will draw two lines representing the feet of Sri Narayana, and in the middle they will put a red line to represent Lakshmi Devi. Because the Sri Vaishnava sampradaya begins with Sri Lakshmi Devi, and they approach Narayana only through Lakshmi, their tilak reflects this process of surrender. Using mud also makes us reflect that we come from clay and go back to clay.

The tilaks of each sampradaya actually depict the siddhanta of the sampradaya.
The scriptures say that a Hindu without tilak is worthy of condemnation and is compared to intellect without clarity

Women wear bindi traditionally on the forehead as a red dot. 'Bindi' is derived from the Sanskrit word 'bindu' or a drop, and suggests the mystic third eye of a person.It is applied as an ornamental mark on the forehead between the two eyebrows — a spot considered a major nerve point in human body since ancient times. The bindi is believed to prevent the loss of "energy", as well as bringing spiritual protection against demons or bad luck.
The red 'kumkum' between the eyebrows is said to retain energy in the human body and control the various levels of concentration. It is also the central point of the base of the creation itself — symbolising auspiciousness and good fortune. Traditional bindi is red in colour. Now women try out all sorts of shapes and designs !