The most important thing this family does in the history of religion is provide a devotional literature that is not in Sanskrit and does not depend on it. From roughly the sixth century onwards, poets in Tamil composed hymns to Shiva and to Vishnu in their own language, in the ordinary speech of the country rather than the priestly one, and those hymns were collected.
The Shaiva collection is the Tirumurai, which includes the songs of the Nayanars; the Vaishnava collection is the Divya Prabandham, the work of the Alvars. Both are treated in the south as scripture, recited in temple worship and commented on in the same way Sanskrit texts are, and the Prabandham is sometimes called the Tamil Veda. What that means in practice is that a major stream of Hindu devotion has a canonical language of its own, with its own commentators, its own temples and its own lineage of teachers, and that a picture of Hinduism drawn only from Sanskrit sources will miss it.
The bhakti movements that later spread across northern India in vernacular languages have their earliest sustained expression here. A language family is not usually a religious fact, and in this case it is one. Telugu, Kannada and Malayalam each carry devotional literatures of their own on the same pattern, and the Lingayat tradition of Karnataka, whose vachanas were composed in Kannada from the twelfth century, held explicitly that the vernacular was the right language in which to approach God and the priestly one was not.
Sources Hymns for the Drowning: Poems for Vishnu by NammalvarNammalvar, translated by A. K. Ramanujan with Speaking of SivaA. K. Ramanujan (translator)