Rigveda 1.1 owes part of its importance to its position as much as to its content: it is the hymn Vedic tradition itself places first among the more than one thousand hymns of the Rigveda, the oldest of the four Vedas and, by most linguistic dating, among the oldest continuously transmitted religious texts in any Indo-European language. Its opening word, agnim, accusative of Agni, is the first word of the entire collection, and later ritual and grammatical tradition treated that placement as meaningful rather than accidental, reading the choice to open the Rigveda with an invocation of the sacrificial fire itself as a statement about what the whole collection exists to serve, correctly performed sacrifice, before the collection moves on to hymns addressed to Indra, Varuna, the Ashvins and the rest of the Vedic pantheon.
The hymn's use as an actual liturgical opening for the fire offering, rather than only as literature to be studied, rests on the same logic: reciting the invocation of Agni at the moment a new sacrificial fire is kindled repeats, at the scale of a single rite, the same opening gesture the Rigveda's own compilers made at the scale of the whole scripture, praising the fire before turning to any of the specific requests or larger hymns that the fire is then asked to carry onward to the gods it serves.