Agni is each the sacred fireplace along with the deity of that fireplace who transmits oblations to another deities. Therefore, this mantra is recited even though invoking the deity Agni in the form of fireplace as witness into the vow. Agne vratapate vrataṃ cariṣyāmi (Vājasaneyī-Saṃhitā in the Śukla [White] Yajurveda https://phrasedirectory.com/listings13312735/little-known-facts-about-baglamukhi