In both Zoroastrianism and Vedic Hinduism, Fire is one of the most important mediums and or Gods that interact with mankind directly with the Invisible. This medium as the other elements are. Here in this understanding of things, fire acts as way of communication between the Invisible and the Physical Reality. The Vedic Hindu version of fire is different than Zoroastrian, Vedic Hindu see the smoke rise from the fire and that is Agni delivering the fire sacrifice and the request offer up to the gods. The Zoroastrians see fire as it purifies metal to create tools and weapons. When you apply the fact that Hinduism is thought to be thousands of years older than Zoroastrianism, it makes more sense from a tribal/village understanding versus a bronze/iron understanding.
Agni (Sanskrit: अग्नि Agni), pronounced ” ăgˈnē “, is a Hindu deity, one of the most important of the Vedic gods. He is the god of fire <energy>  and the acceptor of sacrifices for onwards conveyance to other deities. The sacrifices made to Agni go to the deitiesbecause Agni is a messenger from and to the other gods. He is ever-young, because the fire is re-lit every day, and is immortal. In the Rig Veda (I.95.2), a Rishi prays – for the ten eternal powers to bless Tvashtr (the supreme mind which creates all things) with the birth of Agni which is a reference to the ten undisclosed powers that nourish Agni. Yaskacharya explains that the fire-god is called अग्नि (Agni) because he is अग्रणी (Agrani), the forward leader who is the ever awake disseminator of knowledge and the first principle of thought which manifests as Speech; it is carried at the front in all ritualistic undertakings (yajnas). Pippalāda, the sage of thePrashna Upanishad, merely highlights the एकायुः (the Sole person) status of Agni when he tells Kābandhi Katayāna – ” That very one, Surya who is Aditya, rises up who is Prana and Agni, who is identified with all creatures and who is possessed of all fame. ” The Vedic Rishis knew knowledge to be the quality of the Atman. Surya, Aditya, Prana and Agni stand for the Atman who reveals itself as knowledge by the all-illuminating bright rays of light and who reveals itself as objects cognized by the mind and described through speech (Rig Veda X.135.7). According to the Puranas, the origin of Krittika nakshatra (the Pleiades star-cluster) ruled by Agni, and the birth of Kartikeya is associated with Agni. The Death-conquering Agni-rahasya vidya, which was received by Prajapati from the self-existent Brahman, is detailed in the tenth kanda of the Shatapatha Brahmana. During Vedic times, animal sacrifices to propriate Agni were frequently made. Agni is also referred by the name Chagavahana.
In general terms, Agni is regarded, along with earth, water, air, ether, time, directions, atman and mind, as a thing that exists possessing distinct qualities – पृथिव्यापस्तेजोवायुराकाशं कालो दिगात्मा मन इति द्रव्याणि | – Vaiśeṣika Sūtra (I.i.5)
The Gathic role of atar as the medium for detecting guilt is not directly evident in the later texts of the Avesta, but reappears in modified form as an allegory of burning and annihilating the Angra Mainyu through righteousness, “where Asha Vahishta is identified at times with the household fire on the hearth.” There, “identification in the realms of matter and of spirit serves only to bring more into prominence the main tenets of Zoroaster’s teachings in regard to Asha” (Dhalla, 1938:170). A vestige of the ancient institution of ordeal by heat is nonetheless present in Vendidad 4.54–55, where speaking against the truth and violating the sanctity of promise is punishable by flogging and is detected by the consumption of “water, blazing, of golden color, having the power to detect guilt.” The Zend translation/commentary on this passage translates “blazing” as having brimstone and sulphur, and notes that innocence or guilt was established by the consumption of this “guilt-detecting liquid”. Similarly, in the Denkard, Adharbad Maraspand—the Sassanid era high-priest to whom the collation of the Avesta texts is attributed—is purported to have nine measures of “unburning molten zinc” applied to his chest as proof of accuracy of the sacred texts.
Seen chronologically, the transition from atar as a vehicle of judgement to Atar Yazata the divinity presiding over blazing fire is abrupt. While the older Gathic Avestan texts have heat (and thus fire) associated with harsh judgement, the Younger Avestan texts have the divinity Atar completely representing and being represented by fire itself; and associated with warmth and light and essential for growth. Asha Vahishta‘s association with atar is however carried forward, and they are often mentioned together (Yasna 62.3,Nyashes 5.9, etc.). So also in their roles as protectors, for “when the Evil Spirit assailed the creation of Good Truth, Good Thought and Fire intervened” (Yasht 13.77)
It is in the later texts that Atar is personified as “the son” of Ahura Mazda (standard appellation, Yasna 25.7 et al.) and is addressed as “full of glory and full of healing remedies” (Nyash 5.6). In Yasna 17.11, Atar is “master of the house”, recalling the role of the hearth fire in the Gathas. The same passage enumerates the “five kinds of fire”:
- atar berezi-savah, “the highly beneficent atar“, qualified in Zend texts as “the fire that eats food but drinks no water”, and the kind of fire that burns in an Atash-Behram, the highest grade of fire temple.
- atar vohu-fryana, “the atar of good affection”, later qualified as “the fire diffusing goodness”, and “the fire that consumes both water and food”.
- atar urvazishta, “the atar of greatest bliss”, later qualified as “the fire of happy life”, and “the fire that drinks water but eats no food”.
- atar vazishta, “the atar most swift”, later qualified as the fire in clouds, i.e. lightning, and as “the fire that neither drinks water nor eats food”.
- atar spenishta, “the atar most holy”, described in “Zend” texts as “the fire of prosperity” and as the spiritual fire burning before Ohrmuzd.
The description of the fires in the Sassanid era commentaries (the Zend texts) differs slightly from those described in the Bundahishn (“Original Creation”, completed in the 11th or 12th century). In the latter, the description of the first and last kind of fire is reversed.
Also, the spiritual understanding on how the fire is applied to the individual person is a religious or spiritual sense. When the term religious is used in these religion, its an adjective to describe an interaction with the Invisible. In other words, what we westerners call a spiritual experience is what easterners call religious experience. To them they practice spirituality to receive religious knowledge, not vice versa. Here in also the major difference between the 2 ideas of how interaction is done with the invisible.
Agni is the Tattva of the Manipura Chakra. The solar plexus chakra, is where the center body and is the house where human prana emanates from, is the seat for our desires, and is recharged by the Sun. This is the body’s way of recharging, just as the temple fires’ are lit again every morning. Fire is both destructive and creative in transforming things like cooking or lighting a candle. It also helps you to burn through your limitations and puts lite in the darkness you find yourself in.
It comes down to the pursuit of how one sees and uses fire. Is fire a vehicle to worship and exchange energy with Daeva, to change yourself internally, drive your desires, and used as the Black Flame, or is it a painful judgement used to control your animal nature to the point of twisting your mentallity into believe some “prophet’s” established truth? Think of Atar like a cattle prod to train into the game of the righteous enslavement. Then think of Agni as an advoctate between you and the freedoom of godhood. Fire is with or against you!