Semitic is one branch of Afro-Asiatic and is studied as though it were a family in its own right, with its own comparative grammar, its own journals and its own reconstructed proto-language. There are good reasons for that. It has the longest and densest written record of any branch anywhere: Akkadian is attested from the third millennium BCE, Ugaritic and the Canaanite languages from the second, Aramaic, Hebrew, Phoenician and Old South Arabian from the first, and Arabic, Ge'ez and Syriac carry the record forward without a break to the present. A comparative linguist working on most families is reconstructing an unattested ancestor from modern descendants.
A Semitist can watch the changes happen in documents. The branch is also structurally distinctive, built around consonantal roots of usually three consonants whose vowels carry grammatical information, a pattern that shapes the writing systems: the alphabets of the region record consonants and leave vowels to be supplied, which is why the vocalisation of a scriptural text becomes a scholarly discipline in more than one tradition here.
The Masoretes who fixed the vowels of the Hebrew Bible between the seventh and tenth centuries, and the scholars who fixed the canonical readings of the Qur'an, were doing structurally similar work on structurally similar scripts, several centuries apart and in unconnected traditions, because the writing system posed both of them the same problem.