Linked from
The 13 pages that link to Avestan, each with the reason it gives.
AshaNarrower topic: Asha is the Avestan form of a term whose linguistic roots clarify its meaning.
GathasNarrower topic: The Gathas preserve its oldest known form.
ZoroasterRelated: Its archaic form helps situate the Gathas within early Iranian linguistic history.
MithraRelated: The name and earliest descriptions of Mithra survive in Avestan texts.
MazdaismRelated: Its linguistic evidence helps place the earliest layers of Mazdaism.