Why create such a complex cipher?

[dashstofsk]

Manuscript may be something like book cipher

But why the strange alphabet? Encryption using some book cypher could have been made simply with the local language, with no need for an invented script. Also was it really necessary for every word in the manuscript to be written in V, and that none of it could be written in the local language? Even the one word labels of the celestial objects? Did the meaning of every single word have to be hidden from a third party? 36000 words all had to be laboriously book-cyphered?