What is the main role of phoneticians

To study the Phonemes, which are the link between speech & text (script). 
Leaving the Ideogram-as-script languages of Japan& China, most other modern languages (many members of Indo-European languages family) can be graded on a 'phonetic' scale. There are Phonetic, non-phonetic & somewhat phonetic languages. Sanskrit & most of the (Apabhramsa) derivatives are phonetic (like my language Telugu), in that every letter stands alone with its sound (phoneme) that is not modified by the preceding or succeeding letters. For that there need be many letters, combinations & rules like multiplication tables in the alphabet itself. Grammar is really tough. In short (almost) all sounds of voice can be written down as textual symbols. But there are weird phonemes (or sounds) that can't be transliterated in other languages. Examples are "zh" in Tamil (with tip of the tongue suspended in the center of the cavity of palate) & "!"- the clicking of the ! King people in Botswana. Actually it is Tatami & not Tamil. 
At the other extreme, non-phonetic languages draw from a limited set of alphabet & the 'word' is what makes the phoneme. A letter acquires its phoneme from the neighboring letters (some letters are silent even). In between fall Arabic, Persian, Sindhi & Urdu. In fact, Hindi, a derivative of Sanskrit followed its Sanskrit Phonetics. But in the Muslim period of Indian History was influenced by them & acquired semi-phonetic traits as a twin language of Urdu (Urdu is written as Arabic, right to left while Hindi has Devanagari (Sanskrit) script sharing with Marathi & Nepali)