Avestan was an Iranian language in which the earliest Zoroastrian hymns were orally transmitted since 1500 BCE. Due to lingusitic change, fluency in Avestan as spoken a thousand years earlier was deteorating, and hence the need to write the language became increasingly apparent. By the 3rd century CE an alphabet was created to write down the ancient Avestan language.
The Avestan alphabet was modelled on the Pahlavi script, which in turn was derived from Aramaic. Like Semitic scripts, the direction of writing in Avestan is right to left, and the shape of the letters are cursive like those in contemporary Aramaic scripts. However, there are several differences between Avestan, Pahlavi, and Semitic scripts. First, Avestan has a large number of letters. The Avestan language has a lot of consonants, and neither Pahlavi nor Aramaic had enough to represent all of them unambiguously. As the ancient Avestan hymns were to be preserved, more care was given to distinguish minute phonological differences.
It was probably due to this same reason that all vowels are written, and even vowel length is taken into account.
Due to the age of orally Avestan text, it facilitated the study of Iranian and Indic languages. In fact, the oldest Avestan is so similar to the oldest Sanskrit that you can translate text in one language to another by applying few phonological changes. Like so:
Purple is Avestan, Red is Sanskrit. The ə symbol represents the mid central vowel (schwa) like the "e"s in "taken".