The most important religious activity described in the Vedas was the fire sacrifice, known as yajna or homa.
These sacrifices were conducted only by brahmin priests, the ones who were specialists in the use of Sanskrit.
It was absolutely necessary that the brahmins perfectly pronounced every syllable when they called out to the gods during Vedic ceremonies.
Otherwise, if they made any mistakes in speaking, the ceremony could turn out to be useless. Therefore, brahmins have all along been necessary for the successful enactment of Vedic (and later scriptural) Hindu ceremonies.