The vocalic r is a vowel that sounds like a consonant, and the Indian analysis is unambiguous that it is the former. It carries a syllable on its own, it takes the same short and long pairing as the other vowels, and it sits among them in the table rather than among the liquids. Its dependent form is a curl written beneath the consonant, sharing the space under the line with the u marks and with subjoined consonants.
The letter is not marginal. A substantial group of common Sanskrit nouns is built on stems ending in this vowel, including the kinship terms and a large class of agent nouns, and their inflection is one of the first paradigms a student of the language meets. The vowel also stands in the root behind the word for a seer and behind the name of the oldest of the Vedic collections, so the sign is met early and often in any text with a Vedic frame of reference.
Its pronunciation is where the tradition and modern practice part company. The classical description is of a single vowel sound, an r held as the nucleus of a syllable. Modern readings across India render it as ri or as ru depending on region, and transliteration conventions follow one or the other, which is why the same word turns up in Latin letters in more than one spelling. The Siddham letter is indifferent to all of this. It writes the vowel, and the reading is supplied by whoever is reading.