Records stored in Granite Mountain Records Vault include genealogical and family history information contained in over 2.4 million rolls of microfilm and one million microfiches. This equals about three billion pages of family history records. About 40,000 new microfilm rolls are added to the vault each year.

that seems like a lot of microfilm. my genuine guess? There's probably a wide variety of physics, medicine, and equipment texbooks alongside all the geneology stuff. There's a pretty strong survivalist undercurrent in Mormonism, and it hinges on the church being the only institution to to survive insert any given collapse narrative here. Having a library of useful microfilm under a mountain seems like the sort of generic capital-p Prepared that would appeal to members and Church leadership.

My joke guess? After failing to find archeological evidence of iron tools and horses in the pre-collonial Americas, the church launches a secret intuitive to manufacture their own proof...