The answer is -- nothing. Mary was in need of a Savior as much as anyone, as Harley referenced.
An excellent book is "The Cult of the Virgin Mary: Psychological Origins" by Michael P. Carroll. Carroll draws clear comparisons between the veneration of Mary and the pagan worship of the goddess Cybele.
There are also lots of "excellent books" on the Gnostics version of the Gospel and there are also "excellent books" on Mithraism and its parallelism with Christianity. Also, there are other "excellent books" on the Isis Cult that preceded Christianity that talks about a risen god. For whatever your theological slant, there is a book out there that will "prove" whatever you want to your heart's desire.
Regards
Bizarre question. It is ordinarily necessary for salvation to be baptized (John 3:5), obey the commandments (Luke 18:20), do works of charity (Matthew 20:31-46), have total dedication to Christ (Luke 18:23), partake of the Eucharist (John 6:54-55) and confess sins (James 5:16, 1 John 1:9). Praying to Mary is not mentioned among these necessities. Reading the Institutes is not mentioned either.
A Christian is judged by his works (Apocalypse 22:12). This apostolate is open-ended: at no point can one say "I did which was commanded me" (Matthew 25:26-30, Luke 17:6-10). This is why a Christians seeks mentors in his life. Mary and all the saints are there to offer us the example of their Christian lives and to sustain us with their prayers. Is it necessary to look for them and ask their prayers? No; it is merely foolish not to.