What are the reasons for Hindus performing ceremonies with Agni as a witness - Agni Sakshi?

 

agnisākṣi (अग्निसाक्षि) - any agreement concluded orally, with agni(fire) as a witness.

The agreement concluded can be a marriage, a political agreement, etc.

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1) In Valmiki Ramayana marriage of Sita and Shri Rama was performed keeping fire for a witness/in the presence of Agni or fire - Agnisākṣi (अग्निसाक्षि).

दर्भैस्समैस्समास्तीर्य विधिवन्मन्त्रपूर्वकम्।

अग्निमादाय वेद्यां तु विधिमन्त्रपुरस्कृतम्।।1.73.24।।

जुहावाग्नौ महातेजा वसिष्ठो भगवानृषि:।

Brilliant and worshipful rishi Vasishta chanted, mantras according to tradition, put the darbha grass of equal proportions around the altar, placed the fire on the altar, recited the mantras from the scriptures and offered oblations into the flame.

ततस्सीतां समानीय सर्वाभरणभूषिताम्।।1.73.25।।

समक्षमग्ने स्संस्थाप्य राघवाभिमुखे तदा।

अब्रवीज्जनको राजा कौसल्यानन्दवर्धनम्।।1.73.26।।

Thereafter king Janaka brought Sita adorned with various ornaments and placed her in the presence of Agni in front of Rama, the enhancer of the joy of Kausalya. And said:

इयं सीता मम सुता सहधर्मचरी तव।

प्रतीच्छ चैनां भद्रं ते पाणिं गृह्णीष्व पाणिना।।1.73.27।।

"This my daughter Sita will be your partner in performing her rightful duty. Accept her. Farewell. Take her hand into your own."

पतिव्रता महाभागा छायेवानुगता सदा।

इत्युक्त्वा प्राक्षिपद्राजा मन्त्रपूतं जलं तदा।।1.73.28।।

By being a devoted wife this highly fortunate Sita would always follow you like a shadow". Thus spoken, he sprinkled the water sanctified with mantras (on them).

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2) Shri Rama and Sugreeva take the oath of friendship before an altar of fire .

ततो हनूमान् संत्यज्य भिक्षु रूपम् अरिन्दमः || ४-५-१३
काष्ठयोः स्वेन रूपेण जनयामास पावकम् |
दीप्यमानम् ततो वह्निम् पुष्पैः अभ्यर्च्य सत्कृतम् || ४-५-१४
तयोर् मध्ये तु सुप्रीतो निदधौ सुसमाहितः |

Then the destroyer of enemies Hanuma discarding the guise of ascetic assumed his original monkey form, and on producing fire with two sticks then made it to glow. decorated, worshipped with flowers, then gladly and devoutly placed that fire in between Rama and Sugreeva.

ततो अग्निम् दीप्यमानम् तौ चक्रतुः च प्रदक्षिणम् || ४-५-१५
सुग्रीवो राघवः च एव वयस्यत्वम् उपागतौ |

Then those two performed circumambulations to that well glowing ritual fire, and thus, Rama and Sugreeva entered into the pact of friendship.

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A question may arise as to why only Agni or fire is made a witness to the oral agreement made, but not any God, viz., Vishnu, Shiva, etc?

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According to my understanding, this tradition of doing things with Agni as witness might have commenced in Rig Vedic period itself.

Though Agni was called havyavāhana - हव्यवाहन (the bearer of oblations offered in a Yagna), in Rig Veda, it should be understood that oblations offered in a Yagna are related to MIND only. Those are not physical oblations offered through physical Yagna, but are related to any type of dhyAna or vichAra done through Mind.

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Agni is physical fire as well as the all pervading chaitanya - DIVINE CONSCIOUSNESS. This chaitanya will be experienced ONLY by the SELF REALISED souls or rishis.

Rig Veda 1.1.1 eulogises AGNI as

a) पुरोहित - purohita, ie., the ONE that emerged first in the UNIVERSE,ie., the BRAHMAN or the God.

अग्निमीळे पुरोहितं यज्ञस्य देवमृत्विजम् ।

होतारं रत्नधातमम् ॥

पुरोहित - purohita;; puras + Ahita = FIRST BORN

b) ॠत्विजम् (ṛtvijam)

Shri Aurobindo gave a different perspective to the word ॠत्विजम्

ॠत्विज् (ṛtvij) is a very old word compounded in ancient Sanskrit before the creation of the modern rules of Sandhi, and is composed of ऋत् (ṛt), truth, and विज् (vij), ecstasy or ecstatic.

It means one who has the ecstasy of the truth or सत्यम् (satyam).

Rig Veda 2.1.1 eulogises AGNI, as pervading in HUMANS, TREES, WATER, STONES.

त्वमग्ने द्युभिस्त्वमाशुशुक्षणिस्त्वमद्भ्यस्त्वमश्मनस्परि ।

त्वं वनेभ्यस्त्वमोषधीभ्यस्त्वं नृणां नृपते जायसे शुचिः ॥

You, Agni, (are born) throughout the days, you who are eager to blaze here; you (are born) from the waters, you from the stone, you from the trees, you from the plants, you, men-lord of men, are born blazing.

Rig Veda 2.1.2 to 2.1.16 eulogises AGNI as the BRAHMAN, states AGNI is Indra, Vishnu, SavitR, Varuna, Mitra, Rudra, etc, besides stating that AGNI is Saraswati, ADITI, etc.

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We have to remember that Indra, Vishnu, Varuna, etc the epithets of the same BRAHMAN.

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Hence, AGNI is physical FIRE as well as the BRAHMAN or chaitanya that pervades in all living and non-living things.

As far as I understood, this is the reason as to why the rishis used commence in the presence of AGNI - agnisākṣi (अग्निसाक्षि), ie., in the presence of THE BRAHMAN.