What is the meaningfulness of citing Mantras in Havan?

While doing ‘Agnihotra’ or ‘Havan’, certin mantras / shalokas of Vedas or other religious books, which are in accordance with teachings of Vedas are recited. These mantras / shlokas contain prayers and appreciation (stuti) of God. As such, in Arya culture, day used to be  begun with purifying the atmosphere through putting good things in fire (through ‘Havan’) and with prayers for power of our body & soul to God (through recitation of mantras / shalokas).