A chung, the small a, is the twenty-third of the thirty consonants and the most purely orthographic letter in the alphabet. As an initial it is a weak consonant with barely any independent value; its real work is done in three other positions.
It is one of the five prefixes, standing silently in front of a root letter. It is one of the ten suffixes, closing a syllable, where in the modern language it is generally heard as a lengthening or a quality of the vowel rather than as a consonant. And in the transcription of Sanskrit it is written beneath a letter to mark a long vowel, which is how Tibetan writes a distinction its own language does not make.
It also has a role in the verb. A number of Tibetan verb forms are distinguished by a prefixed small a, so the letter carries grammatical information in a system where very little is marked by anything audible.
For a reader the practical point is that a small a in a syllable is almost never the root letter, and identifying it correctly is a precondition of reading the syllable at all, let alone finding the word in a dictionary.
In transliteration the small a is written with a preceding mark to keep it distinct from the great a. That is one of the places where a transliteration has to carry a distinction a phonetic rendering would simply lose, and it is a fair illustration of why scholarly writing on Tibetan uses one rather than the other.