Why it is so important?
Kumbh is an abode of Sea, Earth and all 4 Vedas : Sages, Saints and Brahmins have honored Kumbh as a symbol of greatness of Deities, natural wealth, materialistic prosperity, grace of Sree and Lakshmi and a storage of knowledge and science. As per the science, Kumbh is an abode of Deities. In the opening of Kumbha, there is abode of Bhagavan Sri Vishnu; in its neck, abode of Sri Mahadev and at its base, there is abode of Sri Brahmadev. It is also stated that in the centre of Kumbh, there is abode of all Deities, all Seas, Mountains, Earth and 4 Vedas; therefore, Kumbh has religious and spiritual importance.
Importance of Kumbh as per the Puranas
War was fought by Devas (Gods) and Asuras (Demons) at the time of Samudra-manthan for obtaining the kalash of nectar : There is history of war between Devas and Asuras hidden in the story related to Kumbh in Puranas. 14 precious stones/ things were obtained during Samudra-manthan (churning of the Sea) in which was included this kalash (Kumbh) with nectar. War was fought between them to obtain this Kumbh. During Kumbh-parva, this Kumbh is also remembered.
Sacred Bath in Kumbh Mela
Bathing in the holy river on the auspicious occasion of Kumbh Mela is the most important activity for millions of people in India. A large tented city is erected and pilgrims stay at tents owned by Pandas (religious and spiritual guides) and at various ashrams. Others will just camp on the ground or turn up for the actual bathing day. Some of these bathing days are designated "royal," and it is on these days that the naga sadhus (naked mendicants) parade and bathe. On other days there will still be people bathing and other events and random processions.
Rituals Performed at the Kumbh Mela
The main ritual performed at that Kumbh Mela is the ritual bath. Hindus believe that submerging themselves in the sacred waters on the most auspicious day of the new moon will absolve them and their ancestors of sin, thus ending the cycle of rebirth. Pilgrims start lining up to bathe from around 3 a.m. on this day.
As the sun comes up, the different groups of sadhus move in procession towards the river to bathe. The Nagas usually lead, while each group tries to outdo the others with more grandeur and fanfare. The moment is magical, and everyone is absorbed in it.
After bathing, the pilgrims wear fresh clothes and proceed to worship by the river bank. They then walk around listening to discourses from the various sadhus.