The religious significance of this grouping is that it holds a devotional literature composed outside Sanskrit and never dependent on it. From around the sixth century, Tamil poets sang to Shiva and to Vishnu in Tamil: the Nayanars on one side, the Alvars on the other, and their hymns were later gathered into the Tirumurai and the Divya Prabandham. Both collections are treated as scripture in the south, recited in temple worship and commented on by named teachers in an unbroken line, and the Prabandham is called the Tamil Veda by the tradition that uses it. This is where the bhakti impulse that later spread across northern India in local languages has its earliest sustained expression.
The Lingayat or Virashaiva tradition of Karnataka made the point explicitly in the twelfth century, composing its vachanas in Kannada and rejecting the priestly monopoly on access to God along with the language that carried it. A picture of Hinduism assembled only from Sanskrit sources will miss all of this, and will also miss how much of what it does describe was shaped by contact with it. The temples themselves are part of the record. The great southern temple complexes, with their gopurams and their endowed festivals, were built and maintained by dynasties who recorded their gifts in Tamil, Telugu and Kannada inscriptions on the walls, so the epigraphy is a religious archive as well as an administrative one.
Sources The Tamil Veda: Pillan's Interpretation of the TiruvaymoliJohn Carman and Vasudha Narayanan with Poems to Siva: The Hymns of the Tamil SaintsIndira Viswanathan Peterson